28 SEGMENTATION AND FORMATION OF THE LAYERS. 



sections through a rather later stage, in which the gastrula 

 cavity is established. In fig. 23 especially, the gut is seen to 

 be traversed by a protoplasmic reticulum containing a nucleus, 

 and the blastopore itself to be partially choked up by a similar 

 reticulum. The latter feature is also seen in fig. 24 b, a 

 section of a slightly older embryo, and, indeed, is characteristic 

 of all the. later gastrula stages until the definite division of the 

 blastopore into the primitive mouth and anus. The gut 

 vacuole, soon after its appearance, acquires an opening to the 

 exterior through a point on the surface where the ectodermal 

 nuclei are and always have been absent. 



The Various Forms of Nuclei in the Early Stages of 



Development. 



I have no observations on the nucleus of the ripe ovum. 

 The facts which I have to record on the structure of the 

 nucleus after the entrance of the spermatozoon may be 

 described under the following heads : 



1. The nucleus of the unsegmented ovum after the conju- 

 gation of the male and female pronuclei of the ectoderm cells 

 in the early stages of segmentation. 



2. The nucleus of the ovum before this event, but after the 

 entrance of the spermatozoon. 



3. The nucleus of the ectoderm during the segmentation 

 and gastrula stages. 



4. The endodermal nuclei. 



1. The nucleus of the completely fertilized ovum and its im- 

 mediate descendants is so large and favorable for study that 

 I have decided to describe it first. It varies considerably in 

 shape aud structure in different ova. These variations no 

 doubt represent different phases in the life-history of the 

 nucleus. It has been impossible for me with the small 

 number (ten) of unsegmented ova at my disposal to determine 

 their sequence. I have, however, seen it in four conditions. 



