42 SEGMENTATrON AND EOEMATION OF THE LAYERS. 



but a multiplication of the nucleus or centre of force whicli 

 causes a corresponding readjustment in the density of the 

 network at different parts of the ovum^ but no break in con- 

 tinuity. 



The Gastrula arises by a process of epibole and is at first 

 solid. 



The endoderm masses at first have no nuclei. Nuclei first 

 appear in them during the progress of the epibole by which the 

 gastrula is formed. I have not been able to determine the 

 origin of these nuclei. They either arise de novo in the 

 endoderm masses or migrate into the latter from the ectoderm. 

 The protoplasmic network at the centre of each endoderm 

 mass is denser than at the periphery, but is without the 

 chromatin granules, so characteristic of a nucleus. But I have 

 described a stage of the nucleus in the fertilised unsegmented 

 ovum in which the chromatin granules are almost entirely 

 absent, and in which the network presents no essential difference 

 from the surrounding network. Again, another in which the 

 nuclear network merges so gradually into the surrounding net- 

 work, that it is impossible to point to any limit between them. 

 I therefore think it quite possible that this central denser 

 protoplasm in the endoderm masses may give rise to the nucleus 

 which subsequently appears. 



The gastrula is a syncytium ; the ectodermal nuclei are 

 arranged around the periphery of the ovum, while the endo- 

 dermal nuclei are within. The latter are characterised by 

 their angular shape, and by never presenting the karyokinetic 

 figures characteristic of the ectodermal nuclei. The protoplasm 

 of this syncytium is much vacuolated throughout, but the 

 vacuoles are largest in the centre. These central vacuoles 

 unite and give rise to the gut cavity, which opens to the 

 exterior through a point on the surface where the ectodermal 

 nuclei have always been absent. This opening is the blastopore. 

 The blastopore, until quite late in development, is traversed by 

 protoplasmic strands, which anastomose with similar strands 

 projecting from the protoplasm lining the large central vacuole 

 or gut. 



