52 LABORATORY STUDIES IN ZOOLOGY 



cells. This first cleavage plane may be spoken of as extending 

 in a vortical or meridional direction. 



3. Four-celled Stage. — The next cleavage plane also extends 

 in a meridional direction but is at right angles to the first, result- 

 ing in four more or less equal cells. 



4. Eight-celled Stage. — The third cleavage plane is said to be 

 equatorial, although appearing nearer the animal pole. It is at 

 right angles to the other two. The result is four smaller cells 

 in the animal hemisphere and four larger ones in the vegetal 

 hemisphere. This inequality in the size of the blastomeres 

 (cells) in the two hemispheres is the result of a relatively small 

 amount of cytoplasm and large amount of inert yolk material 

 in the vegetal hemisphere as compared with the same components 

 of the animal hemisphere. 



5. Sixteen-celled Stage. — At this stage in the development of 

 the egg two vertical grooves at right angles to one another appear 

 simultaneously, thus indicating the directions of the next cleavage 

 planes. These grooves first cut the 4 cells of the animal pole 

 into approximately equal halves, forming a 12-celled state, and 

 then continue through the cells of the vegetal hemisphere to 

 complete the 16-celled stage. Either the 12- or the 16-celled 

 state may be found in the material. 



6. Crescentic Groove Stage. — From this stage on, the inequal- 

 ity in the size of the cells of the two hemispheres coupled with 

 other tendencies results in irregularities in the number and shape 

 of the cells. These later stages are marked by the appearance of 

 the segmentation cavity within. The hlastida differs from that 

 of the starfish in the presence of much thicker walls on the vegetal 

 side. The thicker walls of the vegetal side comprise large yolk- 

 laden cells. As a result, the process of gastrulation is very much 

 modified in the frog from that exemplified by the starfish. The 

 large accumulation of yolk at the vegetal side of the blastula 

 prevents the invagination of this region from taking place so 

 that the gastrula is formed partly by a process of ingrowth and 

 partly by the growth of the animal pole over the vegetal pole. 

 The ingrowth and overgrowth take place more on one side of 

 the egg than on the other and are indicated externally by the 

 appearance of a crescentic groove. This groove marks the 

 advancing edge of the down-growing cells of the animal pole. 

 The crescent represents the beginning of the blastopore and with 

 its appearance the development passes into the gastrula stage. 



