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structure, as well as in most chemical and physical properties, those of the Amphiuma; it is only in size that a very considerable difference exists between them. For while in the Amphiuma the length of these bodies ranges from about 180 to 188 mm., they only attain a length of 130 to 35 mm. in the Frog. And even in the latter their size depends to a certain extent upon the average size of the particular species. Thus I have found the largest corpuscles in the blood of those giants among the frogs (Fig. 48, a and b), the bull-frogs, which attain a length of from twelve to thirteen inches, measured from the nose to the toes.

The difference of colour which I described to exist between the border and the rest of the blood-corpuscle of the Amphiuma, may be equally observed, not only on the blood-corpuscles of the Frog and other Amphibia, but also on those of Birds. While the main part of the corpuscle, as will be remembered, is of a dirty yellow, the border is distinguished by a greenish tint; and although there exists no distinct line of demarcation between the two colours, representing an inner contour, this nevertheless appears under certain conditions, just as in the case of the blood-corpuscles of the Amphiuma. The various changes of form which we have been noticing to take place in the latter in their fresh condition, and which depend on a contraction of the protoplasm, do not occur to such an extent on the fresh blood-corpuscles of the Frog. Not unfrequently, however, corpuscles are met with, in which by a contraction of the protoplasm this part of the corpuscle has in some places become separated from the membraneous layer. Here a vacuum is left between the latter and the retracted protoplasm (Fig. 49), in consequence of which the membraneous layer manifests itself by a double contour. By an oblique illumination the inner contour, representing the place of separation of the membraneous layer from the protoplasm, is rendered darker and more distinct.

This phenomenon, frequently observed on fresh coloured bloodcorpuscles of the Frog, teaches us that, under certain conditions still unknown, a separation of the protoplasm from its membraneous layer may occur, and extend over a greater or smaller space. Now, if such a separation, accompanied by a simultaneous contraction of the protoplasm upon the nucleus, occurs in a number of places, representing regular intervals between a number of small points, at which the union of the protoplasm with its membraneous layer remains undisturbed, that peculiar appearance or star-like form of the blood-corpuscle is produced which has given rise to those theories concerning the structure of these bodies, spoken of before. Such forms are said to be specially obtained in the blood of Amphibia by the aid of water, or by a 2 per cent. solution of boracic acid. Though I have not succeeded in producing these

forms by these means, I have produced them by the action of a very weak solution of chromic acid. But having, moreover, observed them naturally to occur in the fresh blood of the Frog, and also in that of a young Amphibia (Fig. 44), I have taken the view that the reagent plays only a subordinate part, as far as the production of these peculiar forms of blood-corpuscles is concerned; they rather depend on a peculiar condition of the protoplasm, favouring the separation from the membraneous layer. And, indeed, the case appears to me so plain, that I can hardly conceive how it ever could have become a puzzle, even to some eminent histologists. For if we only imagine such a condition to exist throughout, where the protoplasm insensibly blends with the membraneous layer, and that at the same time a contraction of the protoplasm should take place, an entire separation of these two parts of the blood-corpuscle must evidently be the result. The protoplasm in such a case retracts upon the nucleus, which it completely surrounds, while the membraneous layer appears isolated, manifesting itself by a delicate double contour. And again, if the same process should take place without entirely separating the protoplasm from the membraneous layer, but leaving at certain small points a union between the two parts, the result must be the production of a number of filamentary processes, arising from the main bulk of the protoplasm and passing to those points of the membraneous layer. Of course, these processes are drawn out during the contraction of the protoplasm (Fig. 44). In the fresh blood-corpuscles the contraction of the protoplasm must be attributed to natural though at the present time unknown causes. In those instances where the peculiar form of blood-corpuscles is produced by means of certain reagents, I suppose that a condition favouring the separation of the protoplasm from the membraneous layer pre-exists, but that the contraction of the former is called forth by the action of the particular reagent used. A representation of these star-like forms of bloodcorpuscles artificially produced, and on a large scale, will be found in fig. 73, accompanying Rollett's article "On the Blood," in Stricker's 'Handbuch der Lehre von den Geweben, &c.' The double contour, representing the membraneous layer, seems to have been overlooked in this drawing. Fig. 57 of my own drawings represents these forms on a smaller scale, produced on the coloured blood-corpuscles of the Tree-frog (Hyla) by the action of a very weak solution of chromic acid. At a the protoplasm has contracted in the form of a star, while the isolated membraneous layer manifests itself by a delicate double contour; at b the processes of the protoplasm also have, by a continued action of the reagent, become separated from the membraneous layer, and retracted into the general mass of the protoplasm, covering the nucleus.

Among the fresh blood-corpuscles of the Frog, still other forms

than those just discussed are produced by the contraction of the protoplasm, and not unfrequently met with in examining specimens of blood. Thus, blood-corpuscles with irregular sharp ridges or spines, projecting from their surface (Fig. 50), will be observed. It must be obvious to the observer that these forms resemble those thorn-apple forms which are of quite common occurrence in human blood.

When the coloured blood-corpuscles of the Frog are treated with water, the same phenomena are observed as have been described in connection with the blood-corpuscles of the Amphiuma. Their colouring matter gradually disappears, until they are rendered entirely colourless. But their outlines do not become invisible; on the contrary, they appear now in the form of a delicate double contour (Fig. 51, a and b), representing the membraneous layer of the blood-corpuscle. In some instances the outlines appear wavy or crenated; of the probable cause of this appearance I have attempted to give an explanation in connection with the bloodcorpuscle of the Amphiuma. As in the case of the latter, the granules contained in the nucleus are also dissolved by the action of water, while the whole body expands. Treated with a very weak solution of chromic acid after having been acted on by water, the delicate contour of the blood-corpuscle, as well as that of its nucleus, appears at once darker (Fig. 52), while a few granules reappear in the latter.

When the blood-corpuscles of the Frog are exposed to the action of the vapour of acetic acid for about one minute, the protoplasm is observed to coagulate in the form of small granules, while the membraneous layer, represented by its double contour, remains unaltered. At the same time, the former contracts, leaving a clear space between itself and the nucleus (Fig. 53). Treated with the liquid acetic acid in a diluted form, the blood-corpuscles are rendered entirely colourless, but bordered by a delicate double contour; while the nucleus becomes dark bordered.

Subjected to the action of chloroform vapour for several minutes, these blood-corpuscles are discoloured, and their outlines rendered very faint, almost invisible. When in this condition they are treated with a very weak solution of chromic acid, their outlines will reappear in the form of distinct dark double contours, together with numerous granules distributed throughout the interior (Fig. 54, a). In some instances (b and c) a number of these granules are observed to be arranged in lines, radiating from the nucleus toward the periphery. Sometimes the blood-corpuscle assumes the form of a dumb-bell (d).

The effect produced on the blood-corpuscles of the Frog by the action of a strong solution of hydrated chloral, differs somewhat from that observed on these bodies in the blood of the Amphiuma.

The first changes consist in the formation of a number of distinct wrinkles on the surface of the corpuscle (Fig. 55, a), which, however, disappear again with the continued action of the reagent; at the same time numerous granules appear in the protoplasm (b), while the membraneous layer manifests itself by the usual double contour. An interesting specimen with which I met is represented at c; here the membraneous layer of the blood-corpuscle has burst, and the homogeneous protoplasm is seen to escape from the interior. Next, if the action of the reagent continues, the granules in the protoplasm disappear again, the corpuscle appearing perfectly clear (d); and finally, even the membraneous layer is dissolved, so that nothing remains but the nucleus.

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The Coloured Blood-corpuscles of the Tree-frog (Hyla), represented at Fig. 56, a and b, resemble in every respect those of Rana pipiens, except in size; for they only measure about 1 mm. in length. The frequent occurrence of blood-corpuscles with certain portions of their bodies wanting, and appearing as if these had been removed by means of a sharp instrument (Fig. 56, c), has already been mentioned in the beginning of this article, with an attempt to explain this phenomenon. The changes observed to take place in these corpuscles by the action of a weak solution of chromic acid, as represented in Fig. 57, a, b, and c, have also been referred to before.*

The Coloured Blood-corpuscles of Man.-The investigation of these blood-corpuscles, together with those of all other Mammalia, is, in consequence of their small size, rendered much more difficult than that of the large blood-corpuscles met with in the

rum.

* I take the opportunity of mentioning the presence of an Infusorium in the blood of the Tree-frog, which I observed during three successive years in every individual of the species examined. Mr. E. Ray Lankester described a similar animal, in the Quarterly Journal of Microscopical Science' for 1871, which he had discovered in the blood of Rana esculenta, and which he named Undulina ranaAs I am not sure of the exact identity of the animals observed, I do not hesitate to allude to the subject in this place, if only for the purpose of corroborating the fact previously discovered. The Infusorium which I observed (Fig. 58) belongs to the Flagellata, as the long whip or flagellum with which it is provided indicates, and is found in constant motion, alternately coiling its body in one or the other direction into a spiral form. On the convex side of the coil a number of teeth-like processes are observed in constant and rapid motion, giving to this part of the body the appearance of a cog-wheel turning in one direction. As the animal is never seen at rest, but alternately coiling and uncoiling its body, the examination of this part of its locomotive apparatus is rendered very difficult. The motion appears not to be owing to the vibration of cilia; it rather seems to be produced by minute transverse folds successively arising in one direction in the integument of the creature, in the same manner as waves do in the water when a heavy body falls into it. With the object of rendering the motions of the animal slower for observation, I have treated the preparation with several reagents without any advantage; for the animal, in dying, rolls itself up into a confused mass. These Infusoria exist in considerable numbers in the blood of the Tree-frog; a small drop, of the size of a pin's head, when spread into a thin layer under the microscope, may be found to contain about a dozen individuals or more.

animals of the remaining classes of the Vertebrata. While the latter, to which those of the Amphiuma and the Frog, discussed in the preceding pages, belong, are distinguished by their large size, by their oval and bi-convex form, and moreover by enclosing a conspicuous nucleus, giving them the character of a complete organic cell, the blood-corpuscles of the Mammalia are more minute in their dimensions, round and bi-concave in form, and embracing no nucleus, they rather represent an organic cell of a more simple construction. Accessible at all times, the blood-corpuscles of Man, especially, have always been a favourite object for microscopic examination; and owing to the important part which they play in the organism of Man, their nature has been studied and discussed by anatomists and physiologists over and over again. For this reason I should forbear from making any further remarks on the structure and nature of these bodies, if I were not conscious of the fact that, notwithstanding the numerous investigations already made, the subject has not in all cases been fairly dealt with, and is therefore not yet exhausted. On the contrary, in order to finally settle the question whether the coloured blood-corpuscle of Man possesses or not an enveloping membrane, a re-examination of the subject becomes necessary, particularly by those histologists who deny the existence of such a membrane in any form. The coloured blood-corpuscle of Man shows a double contour under various circumstances and conditions, indicating the existence, if not of an enveloping membrane, at least of a membraneous layer on its surface; and anyone who chooses to examine these bodies in the manner to be described hereafter, will be convinced of this fact.

It is an admitted fact that the coloured blood-corpuscles of Man represent minute bi-concave disks (Fig. 59, a and b) with rounded margins, and of a diameter of about mm. They are very delicate in substance, and being elastic and flexible in an unusually high degree, are enabled to resume always their original form, when distorted by mechanical causes. In fact, the momentary changes of form, which they are constantly undergoing when floating in the liquor sanguinis, are owing to the great delicacy and elasticity of their protoplasm. In examining them under the microscope with a sufficient amplification, we observe that the most feeble current arising in the liquid in which they float, disturbs their form, either directly or by causing individual corpuscles to touch each other in passing, or also by calling forth a mutual pressure in a greater number of blood corpuscles. When a coloured blood-corpuscle of Man is examined in the state of rest from the front, its outline appears perfectly round. Being put in the proper focus, so that its outlines appear the most distinct, its centre, to the extent of about one-third of the whole diameter,