218 



PRINCIPLES OF ANIMAL BIOLOGY 



good example. The second cleavage is also meridional and perpendicular 

 to the first plane; four cells are thereby produced. The third cleavage 

 is nearly equatorial, resulting in eight cells. Often, especially when a 

 distinct amount of yolk is present, the third cleavage plane is slightly 

 above the equator, so that the upper four cells are smaller than the lower 

 quartet. 



After the third cleavage there are two or more cleavage planes at the 

 same time. The fourth cleavage passes through two planes, both of 

 them meridional, and perpendicular to one another. The 16 cells thus 

 formed then divide into 32, and so on. Up to the 32-cell stage, in a homo- 

 lecithal egg, the divisions usually take place at the same time in all the 

 cells. Irregularities occur later, some cells dividing earlier and more 



rni 



met 



Fig. 170. — Cleavage in the telolecithal egg of the lamprey. The third cleavage (E) is 

 distinctly above the equator, and the later cells (F, G) are verj unequal in size, em, egg 

 membrane; ma, macromeres; ?m, micromeres (Modified from original drawings by P. 0. 

 Okkelberg.) 



rapidly than others. By this cleavage the single cell (fertilized egg) is 

 converted into hundreds of cells forming a nearly spherical mass. 



At the point to which the process of cleavage has been followed in the 

 preceding paragraphs, the egg, now more correctly called an embryo, is a 

 hollow ball of cells. Often the cells are arranged in a single layer at the 

 surface, sometimes in a few layers, and there is a cavity in the center 

 filled with liquid. The whole embryo is now designated a blastula, the 

 cavity within it the hlastocoele. The blastocoele appears at a very early 

 stage. Even in the four-cell stage, the inner corners of the cells are 

 rounded off, as in Fig. 169, leaving a space. This space is continually 

 present through later cleavages, and becomes larger as segmentation 

 proceeds. The term blastocoele may properly be applied to it in any of 

 these stages. 



