CEPHALOCHOBDA 143 



inner protective envelope. The polar bodies extruded during 

 the ripening of the egg mark the animal pole. Fertilisation 

 takes place by the entrance of a spermatozoon, and the two 

 nuclei fuse to form the segmentation nucleus. Segmentation 

 is holoblastic and simple. Two furrows succeed one another 



FIG. 68. Amphioxus. Segmentation of egg with formation of blastula. 

 After Hatschek. 



through the polar field, and the four cells are converted into 

 eight by a furrow at right angles to the first two and slightly 

 nearer the animal than the vegetable pole. Segmentation 

 proceeds regularly and yields a blastula of one layer of cells, 

 or blastomeres, enclosing a segmentation cavity or blastocoel. 



This is the structure of the Coelenterate blastula, and the 

 characteristic first stage of the Metazoa. The blastula becomes 

 converted into a gastrula, or the two-layered state, by delamina- 

 tion or invagination. If by delamination that is to say, by 

 the cells of the blastula budding cells into the interior which 

 are arranged to form the inner layer the segmentation cavity 

 is converted into the enteron, and an opening is made at the 

 vegetable pole to the exterior, an opening termed the blasto- 

 pore. If by invagination, the segmentation cavity is more or 

 less obliterated by the invagination of the vegetable pole of 

 the blastula, and the opening which results in the process is 

 the blastopore (see fig. 16). In the Coelenterata both methods 

 are to be found. Among the invertebrate Metazoa the invagi- 

 nation method, or a modification of it, is characteristic, and in 

 this respect they are followed by Amphioxus. Gastrulation, 



