100 



ESSENTIALS OF ZOOLOGY 



and the rim on this side, or the dorsal lip, is forced down to the 

 vegetal pole and a little beyond on the other side before gastrulation 

 is complete. The area enclosed by the circle or blastopore is finally 

 very small, and the white vegetal cells which fill it are called the 

 yolk plug. 



While the animal cells have been advancing and covering the 

 vegetal cells, changes have been going on within the egg. The ap- 

 pearance of the groove or dorsal lip of the blastopore was caused 

 by an infolding or invagination of outside cells. This invagination 

 progresses around the egg as the crescent groove extends itself. The 

 cells which consequently come to lie inside are known as endodermal 

 cells, those on the outside as ectodermal cells. 



otic pit 



ectoderm 



rKurftl fube 



mesockrm 



notochord 

 hypochord 

 ^omaiic mesoderm 

 splanchnic mesoderm 



mic^ut 



forejjut 



^^oik 



Action through oiic pit 



Section through mld-^ut 



Fig. 35. — Sections through two levels of the neural tube stage of the developing 

 frog. (Courtesy of General Biological Supply House.) 



By this invagination a cavity known as the archenteron is formed, 

 the walls of which are made up of the invaginated endoderm cells. 

 At first it is quite flat, but it expands as invagination and the other 

 processes of gastrulation proceed, and soon it takes up the space for- 

 merly occupied by the cleavage cavity or blastocoele. Its posterior 

 end is the blastopore, and this, as previously mentioned, is plugged 

 with yolk or vegetal cells known as the yolk plug. As a final result, 

 the gastrula forms a two-layered embryo of ectoderm and endoderm 

 cells, each layer of which may be several cells thick. 



Mesoderm Formation. — Before the process of gastrulation is com- 

 pleted, a sheet of cells forms between the ectoderm and endoderm 



