26 SEGMENTATION AND FORMATION OP THE LAYERS. 



ovum and its products. This process gives rise to the ecto- 

 derm cells. 



Secondly, there is the division of the larger and clearer 

 vegetative part of the ovum into the endoderm masses. This 

 process takes place contemporaneously with the first, but 

 apparently without being governed by the dividing nucleus of 

 the animal or ectodermic part. A.t any rate no part of the 

 latter enters the endoderm masses. It is true that the endo- 

 derm masses in the fresh state do present a central opaque 

 portion (PL I, fig. 8), but I was unable by any of the staining 

 methods I adopted (borax-carmine, hsematoxylin) to find any 

 trace of a structure like an ordinary nucleus in preserved 

 specimens of the segmenting stages, though nuclei were easily 

 visible in the endoderm of the gastrula and later stages. I 

 did find, however, in my stained section of preserved seg- 

 menting ova, that the endoderm masses presented a central 

 portion in which the spongework was much denser than in the 

 peripheral parts (PI. IV, figs. 16, 17). But this central 

 denser portion was entirely without the especially deeply-stain- 

 ing chromatin so characteristic of the ordinary nucleus. This 

 is especially shown by fig. 16. On the other hand, there are 

 in the strands of the network of the endoderm masses small 

 particles of a deeply-staining matter, which are neither visible in 

 the unsegmented ovum nor in the gastrula stages, and which are 

 not to be distinguished from nuclear chromatin. These deeply 

 staining bodies are found in great numbers in the endoderm 

 masses (fig. 16), and to a very small extent in the ectoderm 

 cells. Have these central dense portions of the endoderm 

 masses and the scattered deeply-staining bodies any hand in 

 giving rise to the undoubted nuclei which subsequently appear? 

 In other words, are these structures to be looked upon as 

 nuclei in a condition of structure somewhat different from that 

 usually presented by nuclei ? or are the nuclei of the endoderm 

 cells derived from the nuclei of the ectoderm by migration 

 from the latter at the disco-gastrula stage ? The continuity 

 between the reticulum of the endoderm and ectoderm cells is 

 retained as I have said through the disco-gastrula stage (fig. 



