144 ESSENTIALS OF ZOOLOGY 



it is to be noted, is the process which converts the single- 

 layered blastula into the two-layered condition, into the 

 ectoderm and the endoderm of the Metazoon, and the cavity 

 it contains, bounded by the endoderm, is the rudiment of the 

 alimentary canal, the enteric cavity. 



In the case of Amphioxus, even at the final stage of the 

 blastula the ectodermal cells are small and are called ectomeres, 

 and the endomeres are fewer and larger at the vegetable pole. 



Blastocoel 



Enteron 

 FIG. 69. Amphioxus. Gastrulation. After Hatschek. 



The cells around this patch continue to grow more rapidly 

 than the others, and as a result the vegetable pole becomes 

 flattened and is carried into the interior. The process con- 

 tinues until the endoderm cells apply themselves to the inner 

 face of the ectoderm and the segmentation cavity disappears. 

 The new cavity is the enteron, and its opening the blastopore. 

 This is the gastrula, and it consists only of the two layers 

 mentioned. The blastopore at first is a wide opening, and the 

 enteron a shallow cavity, but the growth at the margin con- 

 tinues to be manifested and in increasing degree. As a result 

 of the multiplication of cells, the blastopore gradually narrows 

 and the gastrula elongates. It thus loses its radial symmetry, 

 and a bilateral symmetry becomes apparent. The growth of 



