194, ADAM SEDGWICK. 
the more complicated and apparently vesicular structure which 
it generally presents is a secondary feature. The fact I refer 
to is this: the first products of the division of the nucleus, 
i.e, the earliest stage of the two new nuclei—I mean the poles 
of the spindle—are simply nodal points around which the 
spongework is radiately arranged, and are without any of the 
complexity of structure which they subsequently acquire.! 
I now pass to the structure of the undoubted endodermal 
nuclei which appear at the disco-gastrula stage. They are 
usually larger than the ectodermal nuclei (see figs. on Pl. XIV), 
and are sometimes very large. They are nearly always of an 
angular shape, and sometimes they are branched. They consist 
of a fine network, which stains, and the strands of which at cer- 
tain points are thickened and give rise to uucleolar-like bodies. 
The strands of the network are continuous with the membrane, 
which is itself continuous with the strands of the extra-nuclear 
reticulum. There is no increase in the density of the extra- 
nuclear reticulum round the nucleus, in fact, rather the opposite. 
These endodermal nuclei appear to divide directly, and they 
never present the figures so characteristic of the indirect divi- 
sion. I have figured on Pl. XII, figs. 4 and 5, some peculiar 
endodermal nuclei found in a young hollow gastrula. Fig. 4 
differs from the ordinary endodermal nuclei in the great deve- 
lopment of its branching processes, which appear to be continued 
into the strands of the extra-nuclear reticulum, and in the fact 
that two of them are connected by processes. Fig. 5 is peculiar 
for the large size, number, and peripheral arrangement of 
the larger staining-bodies. 
Tue STRUCTURE OF THE GASTRULA. 
The fully developed gastrula is, as I have already mentioned, 
a syncytium. Its cavity is a vacuole derived by the enlarge- 
ment of one or the fusion of several of the vacuoles of the 
1 For an account of observations on the supposed spontaneous origin of nuclei 
during development, I may refer to Balfour, ‘Comp. Embryology,’ vol. i (2nd 
ed., p. 108). The ova in all the cases there cited are large-yolked and mero- 
blastic. 
