On the Red Blood-corpuscle of Oviparous Vertebrata. 71 



more distinct. Often some small portion of the edge appears clear 

 before the rest. At the same time the nucleus is seen to be paler 

 than the surrounding substance. Synchronously with this change — 

 and this is noteworthy — the outline of the corpuscle (the " cell-wall ") 

 becomes broader and darker. What was at first a mere edge of 

 homogeneous substance, becomes at length a dark border sharply 

 defined from the coloured matter within. Thus a corpuscle, at first 

 absolutely structureless, homogeneous throughout, is seen gradually 

 to be resolved into central substance or nucleus, external layer or 

 cell-wall, and an intermediate, coloured though very transparent, 

 substance. But — and this is significant — these changes are not 

 always thus fully carried out. It not seldom happens that the nucleus 

 does not appear as a central well-defined regularly oval mass. Some- 

 times it never forms so as to be clearly traced in outline, but remains 

 as an irregular shapeless mass, in its greater portion very obscure. 

 Sometimes only a small part, if any, of an edge can be recognized, 

 most of it appearing to blend indefinitely with the rest of the cell- 

 substance. Sometimes it happens that in many corpuscles the 

 formation of a nucleus does not proceed even so far as this. No 

 distinct separation of substance can anywhere be seen, but shadows, 

 more or less deep, here and there indicate that there is greater 

 aggregation of matter at some parts than at others. Occasionally 

 some of the cells present throughout a granular aspect. I have 

 almost invariably observed, too, a relation between the distinctness 

 of the nucleus and of the cell-wall. "When the nucleus is well de- 

 fined, the cell-wall is strongly marked ; when one is confused, the 

 other is usually fainter. This, however, does not apply to colour ; 

 on the contrary, when the nucleus is least coloured it contrasts 

 most strongly with the surrounding cell. As a rule, the wall of 

 the cell is more strongly marked than the nucleus. 



It will of course be said that the nuclei are present all the while, 

 but are at first concealed by the surrounding substance — the con- 

 tents of the cell. Thus the fact has been accounted for, that the 

 nuclei are not so obvious at first as they subsequently become. 

 But I think a careful comparison of cells will show that those in 

 which a nucleus may be traced are not more transparent than others 

 which are structureless ; and, moreover, when one cell overlaps an- 

 other, the lower one is seen through the upper clearly enough to 

 show that the substance of these cells is sufiiciently transparent to 

 allow of a nucleus being discerned if it exists. When a nucleus is 

 fully formed, it hides that portion of the outline of a cell which lies 

 beneath it. How is it, then, if the nucleus is present from the first, 

 that the portion of the cell over which it subsequently appears is, 

 for a while, plainly seen ? 



The success of the observation is of course influenced by numerous 

 circumstances. Tlie rate at which the nuclei form in the corpuscles 

 varies in different animals. 1 have usually found that in the common 

 frog they are more prone to form than in many other animals — 

 quicker than in most fishes, or even than in some birds. But this 

 does not seem always to depend upon their larger size ; for in the 



