STBUCTUKE OF THE GASTRULA. 37 



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 PI. V), 

 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 nucleolar-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 PI. Ill, 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. 



The 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 

 mass of endoderm. The whole embryo at this stage (PI. 

 V, figs. 24 a — d) is vacuolated, the ectoderm as well as the 



• 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. 



