ONTOGENESIS 



161 



Gastrulation. Differentiation may be said to 

 begin at about this point, although, as we have seen, 

 the undivided zygote, in many cases, is regionally 

 differentiated from the start. When the blastula 

 stage has been reached (in a typical holoblastic 

 egg), one side begins to pit in or " invaginate," 

 as one might indent a soft rubber ball with his thumb. 

 (See fig. 62.) This process continues in some species 



FIG. 62. Cleavage and gastrulation in a holoblastic egg (the pond 

 snail): a, undivided zygote; b, first cleavage; c, second cleavage; d, third 

 cleavage; e, blastula; /, blastula in section; g, beginning of invagination, 

 in section; h, completed gastrula, in section. (From Jordan and Kel- 

 logg, after Rabl.) 



of animals until the invaginated cell-wall comes to 

 lie against the inner side of the rest of the blastula 

 wall, obliterating the blastoccele and forming a two- 

 layered shell surrounding a newly formed central 

 cavity. This cavity, unlike that of the blastula, 

 opens to the exterior at the region of invagination. 

 This stage is called the gastrula, the cavity is called 

 the archenteron, and its opening to the exterior just 



