432 OF NUTRITION. 



slant ; whilst that of the blood-discs is so variable, that the former, though 

 sometimes the smaller, are in other instances far larger than the latter. If it 

 be admitted that the red corpuscles have the power of reproduction, like other 

 isolated cells, it does not seem necessary to seek elsewhere for the source of 

 their constant renewal ; and various facts, hereafter to be stated, appear to the 

 Author strongly indicative of the entire functional as well as structural differ- 

 ence, between the red and the colourless corpuscles of the blood of Vertebrata. 

 575. That the red Blood-discs, when first formed in the embryo, have an 

 origin common to that of all other tissues, cannot be doubted. They are pro- 

 duced, in the embryo of the Bird, in the portion of the germinal membrane 

 which afterwards becomes the area vasculosa ; this consists of delicate cells 

 very uniformly disposed ; and whilst capillary vessels are being formed by 

 the union of the cavities of these, blood-discs seem to be developed from the 

 granules or cell-germs they contain. These changes take place about the 

 second or third day of incubation ; but it is not until some days afterwards 

 that the discs assume their characteristic form.* As at this period no special 

 organs exist in the embryonic structure, it is evident that the blood must be 

 formed by the cells of the germinal membrane, at the expense of the albumi- 

 nous alimentary materials which they absorb from the yolk ; hence we may 



Fig. 102. 



& 



Production of Blood-Corpuscles in Chick, on the fourth day of incubation ; a, particles fully formed : b. 

 particles in progress of formation ; c, similar particles, altered by dilute acetic acid so as to display their 

 nuclei. (After Wagner.) 



infer that no special organ can be needed for this purpose in the adult, and 

 that the assignment of the manufacture of blood-corpuscles by some physiolo- 

 gists to the Spleen, by others to the Thymus, must be incorrect. The cor- 



* Mr. Macleod gives the following history of the development of the blood-corpuscles 

 in the Chick. In blood withdrawn from the heart, on the third day, and diluted with 

 serum, or from the germinal membrane or allantois, and diluted with fluid albumen, 

 *' a number of small granules are seen floating about the field : these enlarge and become 

 clearer in the centre ; this enlargement goes on very rapidly, and when they have gained 

 to about twice their original size, the central clear part becomes dull. This dullness 

 slightly increases, and in a short time it is seen to be distinctly granular: whilst the 

 borders are observed to be well-defined, smooth, and clearer than the central part. The 

 enlargement of these bodies, with the granular appearance of their centre, seems not to 

 depend on the aggregation of granules round a central one, but on a property which 

 they have in themselves of enlarging and presenting that figure. During all this time 

 they are quite spherical and of good consistence, as they do not lose their form by con- 

 siderable pressure. In the second stage, the central portion gradually becomes less 

 opaque, and ceases to appear granular, the external portion at the same separating in 

 some degree from the central part. The blood-corpuscle, in this stage of development, 

 has the appearance of a slightly flattened round cell, formed of a somewhat delicate but 

 elastic membrane, with a nucleus in the centre. At this time a number of these bodies, 

 being close together in the field, present a yellowish colour. The cell is disc-like, rather 

 concave, but the nuclus convex. In the third stage, one side of the corpuscle gradually 

 elongates, giving it a pear-shaped appearance; the opposite side then elongates itself in 

 a similar manner, and to the same degree. The concavity between the nucleus and 

 border disappears, and the whole becomes slightly convex. The hue at the same time 

 gradually becomes redder." (London and Edinburgh Monthly Journal, September, 1842.) 



