l6 EARLY AMPHIBIAN DEVELOPMENT 



and the cells containing more yolk (those of the vegetative hemi- 

 sphere) will necessarily divide less fast than the cells of the animal 

 hemisphere which are relatively free from yolk. Consequently, the 

 cells of the vegetative hemisphere will be larger than those of the 

 animal hemisphere at any given time during cleavage. 



There is also a slight difference in the size of the blastomeres at 

 different positions on the same circle of latitude : a difference which 

 is already shown by the animal hemisphere cells at the 8-cell stage. ^ 

 Though the cause of this size difference at this early stage is ob- 

 scure, at later stages of cleavage it is due to the fact that the cells on 

 the dorsal side divide slightly faster and therefore become a little 

 smaller than those on the ventral side. 



The next stage in differentiation consists in the conversion of the 

 ball of cells — the blastula — into a double-layered sac or gastrula, 

 by means of the process of gastrulation. Owing to the large amount 

 of yolk present in the amphibian egg, this process is not as simple 

 as in other forms (such as Amphioxiis) where gastrulation is a simple 

 invagination of one side of the blastula into the other. In the am- 

 phibian, the same result is achieved by the spreading of the cells of 

 the animal hemisphere and their downgrowth over those of the 

 vegetative hemisphere, at the same time as they tuck in or invaginate 

 and then extend forwards beneath the surface of the outer layer. 

 This process of spreading and growing over (epiboly), and of tuck- 

 ing in (invagination), first takes place on the dorsal side of the 

 embryo, in the region of the grey crescent, and gives rise to a lip 

 known as the dorsal lip of the blastopore. 



Eventually, this lip of overgrowth and tucking in forms a com- 

 plete ring by extending laterally, until the two sides of the lip meet 

 on the ventral side of the embryo. In this way the blastopore be- 

 comes a circular aperture leading into the cavity of the archenteron 

 or future gut. This gut-cavity is a new formation and the direct 

 result of gastrulation. Its lining is made up partly of the cells that 

 have been tucked in round the rim of the lip of the blastopore, and 

 partly of the yolk-laden cells which originally occupied the vege- 

 tative pole of the egg. The amount of these yolk-cells is too large 

 for them to be completely accommodated in the newly formed gut- 

 cavity, with the result that some of them protrude through the 



^ Morgan and Boring, 1903. 



