254 LILIAN SHELDON. 



are a few nuclei present in the central yolk, and these probably 

 are the early endodermal nuclei. At the stage when the 

 embryo is first definitely formed, and lies as a sac within the 

 peripheral layer, there are nuclei present within the body 

 of the embryo; these are irregular angular bodies with a 

 granular structure, scattered irregularly in the central yolk, often 

 containing one or two chromatin particles; their boundary 

 is often indistinct, so that the nuclear passes imperceptibly 

 into the protoplasmic substance. Later, the endoderm nuclei 

 are much more numerous, and are arranged round the peri- 

 phery of the yolk in a regular manner. Since in no case 

 whatever at any stage have I found the least trace of any 

 karyokinetic figures in these endodermic nuclei, and as also I 

 often find in the central yolk all stages, from small masses of 

 chromatin up to definite, large, fully-formed nuclei, I am 

 inclined to believe that they arise by a process of free nuclear 

 formation, and that no nuclear division takes place, at all 

 events, till after the stage at which the endoderm is present as 

 a definite layer at the periphery of the central yolk mass. 



Summary. 



1. The ripe ovum of P. novae-zealandise is very heavily 

 charged with food-yolk, which causes it to be of comparatively 

 large size. 



2. The segmentation is on the centrolecithal type; the 

 protoplasm is mainly at one pole of the egg, and in this proto- 

 plasm nuclei arise, probably by the division of the original 

 segmentation nucleus. The protoplasm forms a loose reticulum 

 containing nuclei on the surface of the egg, which first extends 

 over only a small area, but later spreads over the surface, until 

 in the latest stage which I have, it covers about half the 

 periphery of the egg. 



3. In the latest segmenting ova there are small masses of 

 protoplasm in the centre of the egg ; occasionally one of these 

 may contain a nucleus, and in one case three chromatin masses 

 are present in one of these protoplasmic areas. 



4. Shortly after the segmentation begins the yolk becomes 



