CHAPTER III. 



ON THE CHANGES FROM STAGE A TO STAGE F. 



The Ectoderm, Endoderm, and Early History of the 



Somites. 



The Ectoderm. 



During Stage a, the ectodermal or outer part of the embryo 

 consists of a closely reticulated protoplasm, which contains a 

 single layer of oval nuclei of fairly uniform size. At the lips of 

 the blastopore this layer, with its contained nuclei, is prolonged 

 inwards for a short distance to the more vacuolated endodermal 

 part of the embryo. These nuclei, which are arranged in a 

 layer immediately within the lips of the blastopore, resemble in 

 their shape the ectodermal nuclei (Plate VI, figs. 2, 3), and 

 they may be regarded as ectodermal or endodermal according 

 to the inclination of the observer. I am inclined to call them 

 intermediate or undifferentiated nuclei, that is to say, nuclei 

 which cannot be definitely assigned to the endoderm or ecto- 

 derm, and in this capacity I shall have occasion to return 

 to them when I come to speak of the later history of the 

 mesoderm. 



The protoplasm surrounding these nuclei projects into the 

 cavity of the blastopore and into the archenteron, in the form 

 of delicate processes, which anastomose with other similar 

 processes (Plate VI, fig. 3 ; Plate V, fig. 26 b). 



When the mesodermal bands are formed, the nuclei of the 

 ectoderm overlying them become slightly longer than elsewhere, 

 and the ectoderm has the appearance of being composed of 

 columnar cells. The nuclei are, however, still in a single 

 layer. 



