ITS DIFFERENT PHASES OF DEVELOPMENT IN THE ANIMAL SERIES. 77 
nucleus was circular (fig. 12.). In a few nucleated cells indeed the cellseform nucleus 
was of a somewhat oval shape (fig. 1 7 -), but this did not seem to be more than what 
I had occasionally observed in the blood of those Mammifera, in which the “ red 
corpuscles” are circular. 
85. My reference to the blood of the Paco thus appeared to decide against an 
essential correspondence in form between the cellseform nucleus of the nucleated cell 
and the “red corpuscle.” A little further study of the blood of the Paco however 
soon cleared away this apparent difficulty, and showed me a state of things more 
corroborative of my view regarding the nature of the “ red corpuscle,” and in much 
closer correspondence with analogy perhaps than would have been that which I sup- 
posed possible. I discovered, in fact, corpuscles in different stages of transition, from 
the circular form similar to that of the cellseform nucleus within the nucleated cell, 
to the oval form of the perfect “red corpuscle;” and I also discovered that in these 
different stages of transition in respect of form there are different degrees of colora- 
tion (figs. 26, 2/, 28, 29, 30.). 
86. I had already observed progressive degrees of coloration in the circular “ red 
corpuscles” of the Mammifera generally. Thus, in the blood of the Horse, for example, 
I had observed corpuscles (fig. 19.) similar on the one hand to the “'red corpuscle” 
(fig. 20.) but uncoloured, and on the other to the cellseform nucleus of the nucleated 
cell (fig. 15.), and this in all respects. Such corpuscles I was disposed to regard as 
cellseform nuclei recently set free from the nucleated cells, but not yet reddened by 
the secretion of colouring matter into their interior. Again, I had observed corpuscles 
similar to those just mentioned but presenting a red tinge in the middle, the circum- 
ference untinged appearing like a transparent and colourless ring (fig. 21.) ; but the 
combination of progressive stages of coloration of the “red corpuscle” with its pro- 
gressive changes of form in the blood of the Paco is peculiarly interesting. 
87- The transition from the circular to the oval shape presented by the “red cor- 
puscle” of the Paco, is analogous to that which we have seen presented by the red 
nucleated cell of the oviparous Vertebrata. It is also worthy of remark, that like 
the latter the oval red corpuscle of the Paco is reduced to a circular form by the 
action of water (fig. 31.). The progressive degrees of coloration are also analogous 
to what are presented by the nucleated cells of the blood of the oviparous Vertebrata. 
But it is curious to observe, that whilst among the Mammifera the attainment of the 
oval form by the “ red corpuscle” is exceptional, among the oviparous Vertebrata 
the attainment of the oval form by the red nucleated cell is the rule. 
88. But to return to the bearing of the facts I have now related on the question 
before us. They show that originally the “ red corpuscles*” of the Paco are of a cir- 
cular form like the “ red corpuscles” of other mammifera, but that whilst the latter 
remain circular, the former gradually acquire an oval shape. From this it follows that 
the circular form of the cellseform nucleus of the nucleated cell of the blood of the 
Paco, so far from being an objection to the view of the nature of the “ red corpuscle” 
* In their original circular form they are as yet uncoloured. 
