30 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



gastrula stages, respectively, in the course of their 

 development. The cup-like cavity of the gastrula is 

 known as the archenteron or ccelenteron, and is des- 

 tined to develop into the alimentary canal. The 

 pore or external opening of the coelenteron is called 

 the blastopore. 



The gastrula has two layers of cells : an outer, the 

 ectoderm, and an inner, the entoderm. The cells of 



Segmentation cavity. Ectode 



I 



Blastula. Blastopore gas/rula. 



Fig. 7. Sections through a blastula and a gastrula of amphioxus. 



the entoderm are much larger than the cells of the 

 ectoderm and there is thus a structural difference. 

 In cleavage that results in a two-layered blastoderm 

 the term hypoderm is used, which is thus morpho- 

 logically equivalent to the entoderm. 



The two-layered gastrula is rapidly invaded by a 

 third layer of cells, the mesoderm, which develops 

 between the first two layers and ultimately fills that 

 cavity. This cavity, which is the segmentation 

 cavity of the blastula, permanently disappears. 

 The origin of the mesoderm has long been a con- 

 tested question. The favored theory seems to be 

 that for higher forms, at least, it develops from the 

 hypoderm. 



