1908] Davis. — Life-history of Doh'cJioglossus. 207 



ali'endy noted, my observations on this stage agree with Bateson's 

 account. I have, therefore, omitted many details. 



Mesoderm and body cavities. Bateson (p. 214) says of the 

 two-layer stage just described: "The hypoblast of the middle 

 region is seen to be more eolnnniar in character than that of the 

 anterior region, whih^ the other appearances are the same." Fig. 

 22 shows this condition. It will be noticed that the cells of the 

 extreme anterior are somewhat shortened. The cells at the trans- 

 ition from the anterior to the middle region tend to lose their 

 columnar character and to become irregular. Here, as we shall 

 see, is the beginning of a series of changes in the endodermal 

 cylinder which is to give rise to the mesoderm. 



Since Spengel '04 associates the formation of the mouth with 

 the origin of the mesoderm in tornaria it may be worth while to 

 state that according to Bateson's observations on B. Kowalevskii 

 and my own on D. pusilliis the mouth does not appear until after 

 the mesoderm is formed. 



By the time the two layers in the blastoporic region have 

 become separated, that portion of the endoderm which has just 

 been described as composed of irregular cells (fig. 22) has pro- 

 jected outward and backward somewhat, thus partially separating 

 the archenteron into anterior and posterior parts. The former 

 will be referred to as "anterior body cavity" (Bateson '84, p. 

 216). This stage of development is shown in fig. 23, pi. 8. It 

 wnll be seen that the cells of the walls of the anterior cavity are 

 becoming irregular, especially on the sides, and that some cells 

 are being budded off into the cavity. Rapid growth and redis- 

 position of cells continue until the condition shown in fig. 24 is 

 reached. Here the anterior body cavity although irregular shows 

 the beginning of a lateral and backward projection. The walls 

 of the anterior body cavity and those of the archenteron are 

 sharply differentiated in character of their cells. The archen- 

 teron will henceforth be referred to as entcron. 



The lateral backward projections of the walls of the anterior 

 body cavity continue to grow. The next two stages are shown in 

 fig. 25 and fig. 26, pi. 8. 



It nuist be here noted that the backward growth is not only 

 lateral, but, for a short distance, dorsal as well. There is no 



