SYNCYTIAL NATURE OP THE EMBRYO. 25 



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 m 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 (PI. I, 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 IV, 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. 



We may therefore look upon the ovum of the Cape Peri- 

 patus as presenting two different modes of segmentation, 

 neither of which are instances of complete cleavage in the 

 ordinary acceptation of the term. 



First, there is the segmentation preceded and apparently 

 determined by the division of the nucleus of the fertilised 



