408 DEVELOPMENT OF FROG AND OTHER VERTEBRATA 



which begins as early as the eight-cell stage. Sections of later 

 stages (Fig. 214 D) show the development as cell division proceeds. 

 The fully developed blastula is composed of many small darkly 

 pigmented cells in the animal hemisphere and larger yolk-laden 

 cells in the vegetative region. The roof of the blastocoele is several 

 cells in thickness while the entire vegetative hemisphere is occupied 

 by the larger cells. Such a blastula may be compared with the 

 simpler types found in an animal like amphioxus, if we imagine an 

 increase in the amount of yolk with a resulting enlargement of 

 the cells in the vegetative hemisphere and consequent restriction 

 of the blastocoele to the animal half of the egg. 



The Gastrula. — The gastrula that arises from the blastula of 

 the frog by the process of gastrulation can be better understood by 

 reference to what occurs in an animal like amphioxus (Fig. 209) 

 or the sea urchin, in which the blastula is more nearly a hollow 

 sphere with the cells of the vegetative hemisphere only sHghtly 

 larger than those of the animal region. According to the time- 

 honored illustration, such a blastula becomes a gastrula by invag- 

 ination of the cells of the vegetative portion, as one might push 

 in the side of a hollow rubber ball to form a hemispherical two- 

 layered structure. By a narrowing of its margin, the cup-shaped 

 cavity thus formed becomes connected with the outside by a 

 smaller opening, the blastopore, and the gastrula is produced, with 

 an outer layer of ectoderm cells and larger endoderm cells lining the 

 primitive enteric cavity, or archenteron. Referring again to the 

 comparison between the blastula of the frog and that of amphioxus, 

 it will be seen that such an invagination as occurs in the latter 

 would be mechanically impossible for a blastula, in which the 

 entire vegetative hemisphere is occupied by yolk-laden cells, 

 although some modification of the process might take place. What 

 actually happens in the frog may be characterized as an invagina- 

 tion around one side of the yolk-laden cells, whereby they are 

 enclosed, along with the cavity of the archenteron, by an over- 

 growth of cells of the animal hemisphere. The process is rather 

 compUcated, but its essential features are as follows: There 

 appears, upon the side of the late blastula stage that is destined to 

 form the nervous system (cf. Figs. 213 G and 214 E), a crescentic 

 groove, which may be called the blastopore, since it is the external 

 opening of a cavity that originates by an infolding of the surface 

 and grows inward as the archenteron. As gastrulation proceeds^ 



