22 



STRUCTURE OF THE VERTEBRATES 



After the fourth or fifth cleavage there is a slightly more 

 rapid rate of cleavage in the dorsal hemisphere, and these cells 

 are distinctly smaller. The segmentation cavity has also en- 

 larged, so that at about the 256-cell stage the embryo is a 

 hollow sphere, the blastula. The process of gastrulation begins 

 immediately after, and may be illustrated by pushing a finger 

 into a rubber ball until a cup-shaped, two-layered structure 

 results. In Amphioxus gastrulation is caused by two synchronous 

 processes: (1) the more rapid growth of the dorsal cells causes 

 this hemisphere to become larger than the ventral, and the 

 blastula assumes a hemispherical shape; and (2) the ventral 

 cells invaginate, or push inward, until they come in contact 

 with the dorsal layer. The earlier segmentation cavity is prac- 

 tically obliterated, and a new cavity is formed inside the cup. 

 Thus the early gastrula is two-layered, the outer layer of cells 

 being the ectoderm, the inner layer (derived from the ventral 

 hemisphere) being the endoderm. The inner cavity, surrounded 

 by endoderm, is the archenteron or primitive gut. Continued 

 growth of the gastrula causes it to elongate, and the lips of the 

 cup to narrow toward a central point. The opening left after 



Polar body 



Uncleaved 

 egg 



2 Cells 



4 Cells 



8 Cells ^^ ^^^^^ Blastula 



Fig. 4. Cleavage of Amphioxus from egg cell to blastula. 



