19.22] Phases in the Development of Ciirysemys Cinerea 67 



Cleavage 



Since Chrysemys cinerea deposits its eggs in an advanced stage, 

 considerable difficulty was experienced in finding cleavage stages. 

 Two eggs were taken having sixty-four or more cells arranged in a 

 discoidal plate. In appearance they were similar to those of the 

 pigeon eggs photographed by Miss Blount and published in Lillie's 

 (1908) text, page 45, tig. C. On account of the hardness of the 

 embryos, sections were not secured. 



Formation of Blastula 



The formation of the blastula in Chelonians has been studied Dy 

 Agassiz (1. c), Will (1893) and Mitsukuri (1896). The sections show 

 a layer of rather large cells, co-extensive with the blastodisc, and 

 underlying these, a layer of more spherical cells. The former is at 

 once recognized as the ectoderm and the latter as the yolk endoderm. 

 There is sometimes a cavity present which may be called a segmen- 

 tation cavity. In the earlier works it was called a Baerische Hohle, 

 and was figured by Will (1. c. ). 



Some Observations Upon Gastrulation and the Formation of 



THE Blastopore 



Keibel (1905) raised the question as to the advisability of divid- 

 ing the process of gastrulation into two parts. He suggested that 

 the part involving invagination or involution be considered as the 

 first part and the formation of the mesoderm as the second. This 

 plan has been adopted in this paper, 



Mehnert's work (1892) concerning the process of gastrulation is 

 by far the most complete, and carries an excellent bibliography. 

 Mitsukuri (1893) published a preliminary note upon this process, 

 which was more fully treated by him in 1894. There is nothing 

 unusual in the process. The views of these writers will be given 

 later. 



The earliest stage of gastrulation is shown by the first invagina- 

 tion of the ectoderm to form a small cup. This is commonly known 

 as the blastopore stage. No trouble was experienced in securing it, 

 since by far the greater number of eggs opened immediately were 

 found to be in this condition. Altogether, twenty such embryos have 



