PEIMART ABSENCE OP NUCLEI IN THE ENDODEEM. 27 



16) to the gastrula stage (figs. 20^ 24 — 26) ; indeed, in the 

 gastrula stage it becomes, in consequence of the closer approxi- 

 mation of the endoderm masses to the whole inner surface 

 of the ectoderm cap (fig. 20), still more marked. The strands 

 of the reticulum of the ectoderm cells are continued into the 

 strands of the ectoderm masses, and the whole ovum presents 

 the appearance of a multi-nucleated vacuolated mass (fig. 20). 

 It may be that some of the nuclei of the ectoderm cells pass 

 along these continuous strands into the endoderm. But against 

 this view are these two facts : (1) I have never seen any trace 

 of such a migrating nucleus, and (2) the structure of the 

 endoderm nuclei of the gastrula stage is so very unlike that of 

 the ectoderm nuclei. Compare PI. V, figs. 24 — 26. 



Before leaving this subject, I may call attention to the small 

 bodies present in the endoderm masses in the early gastrula 

 stage (fig. 20). These bodies do not stain so deeply as the 

 endodermal nuclei, which are now present in small numbers, 

 or as the small, deeply-staining bodies seen in the sections of 

 the disco-gastrula stage (PI. IV, fig. 16) ; but they stain 

 more deeply than the ordinary protoplasmic reticulum. Can 

 these bodies have anything to do with the endodermal nuclei 

 which are now appearing? 



This subject is one of extreme interest, and I shall return 

 to a consideration of it when I have described the structure 

 of the nucleus of the unsegmented ovum and its immediate 

 descendants. 



3. The third point of interest in the development of the 

 gastrula is the mode of origin of the cavity of the gastrula. 



The solid gastrula consists of a multi-nucleated, much- 

 vacuolated mass of protoplasm. The gut of the gastrula 

 arises from an enlargement and confluence of the 

 vacuoles in the centre of this mass. The gut of Peri- 

 patus is therefore to be looked upon as a vacuole, resembling 

 in all essential respects the cavity in the body of a ciliated 

 Infusorian. I refer to PI. IV, fig. 20, which represents a 

 section through a gastrula in which the gut is only just 

 appearing, and to PI. V, figs. 23, 24, which represent 



