EMBRYONIC DEVELOPMENT 



201 



ing account of cleavage the egg may be likened to the earth with its two 

 poles, so that a plane passing through the animal and vegetative poles 

 may be spoken of as meridional, other planes aii equatorial or parallel 

 to the plane of the equator. 



In alecithal eggs the early cleavage is very regular (Fig. 172, above). 

 The first cleavage plane is meridional, passing through both animal and 

 vegetative poles and dividing the egg into two approximately equal cells. 



ALECITHAL 

 or 

 HOMOLECITHAL' 

 (SEA CUCUMBER) 



MILDLY 



TELOLECITHAL 



(FROG) 



STRONGLY 



TELOLECITHAL 



(BIRD) 



CENTRO- 



LECITHAL 

 (INSECT) 



Fig. 172. — Cleavage of eggs, in relation to the amount and distribution of the yolk in them. 



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. 



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 such 

 an egg, the divisions usually take place at the same time in all the cells; 

 but irregularities occur later, and some cells divide earlier and more 

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

 converted into hundreds of cells forming a nearly spherical mass, with a 



