182 ADAM SEDGWIOCK. 
of epibole opens to the exterior through the blastopore. The 
ovum has now reached the gastrula stage (vide Part 1, figs. 
19 and 21). 
Before passing on to consider the structure of the gastrula 
and the formation of the mesoderm, I desire to call attention 
to certain remarkable features in the preceding development. 
1. The embryo at the gastrula stage, and in all the 
earlier stages of development, is a syncytium. I 
have already pointed out that the segmentation is not a true 
segmentation. The segments do not separate from one another, 
bnt remain connected by a loose protoplasmic network. What 
happens is this: the nucleus of the fertilised ovum divides and 
gives rise to the nuclei of the two first segments. This causes a 
redistribution in the arrangement of the protoplasmic network, 
but no break in its continuity. In the unfertilised ovum there 
is only one centre—the nucleus—around which the proto- 
plasmic reticulum is especially dense; while in an ovum with 
two segments there are two points—the two nuclei—around 
which we find an especial closeness of the reticulum. In an 
ovum with four segments there are four points around which 
the reticulum presents this especial density, and so on to the 
close of segmentation (Part 1, figs. 1, 4,5). In each case the 
centre is occupied by a nucleus derived by division from the 
nucleus of the fertilised ovum. But this is not all, and I come 
to the second remarkable feature I wish to mention. 
2. No part of the nucleus or centre of force of 
the unsegmented ovum enters the clear endoderm 
masses. Its products remain confined to the ectoderm cells. 
The endoderm masses are, during the segmentation stages, 
without any structure resembling a nucleus as ordinarily 
described, and they do not acquire one till the disco-gastrula 
stage when the endoderm masses are beginning to aggregate 
(Pl. XIII, fig. 16.) The endodermal nuclei, when they do 
appear, differ considerably in structure from the nuclei of the 
ectoderm. They are larger and have a very irregular shape ; 
and further, they do not present the usual karyokinetic figures 
so characteristic of a dividing nucleus, but divide directly. 
