EMBRYOLOCY 



217 



the first maturation spindle. It remains in the oocyte during the rest 

 of the maturation process. In the frog, rabbit, and some others the 

 spermatozoon enters after the first polar l)ody is formed, but before the 

 second. In the sea-urchin the sperm does not enter until the maturation 

 of the egg is complete. 



In eggs that possess a shell at the time of fertilization, there is an 

 opening through which the sperm enters (Fig. 167, m). In naked eggs, 

 the sperm may enter anywhere. Usually only one spermatozoon pene- 

 trates an egg. Some change, probably of a chemical nature, takes place 

 in the protoplasm of the egg when a spermatozoon unites with it, such 

 that no other spermatozoa can be drawn in. When by accident two or 

 more spermatozoa gain entrance at the same time, abnormalities of 

 development are likely to occur. However, in some animals numerous 

 spermatozoa regularly enter the egg; but the nucleus of only one of 

 them unites with the egg nucleus. 



Fig. 169. — Cleavage in the homolecithal egg of the sea-cucumber Synapta. The cells 

 are all of nearly the same size. {Modified from Wilson, after Selenka, courtesy of Macmillan 

 Co.) 



Cleavage. — Shortly after fertilization, within a time measured by 

 minutes or hours in most animals, the fertihzed egg begins to divide. 

 This division, which is repeated in rapid succession until the egg is con- 

 verted into many cells, is called cleavage or segmentation. In the following 

 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 as equatorial or parallel to the 

 plane of the equator. The manner in which cleavage occurs is related 

 to the distribution of yolk in the egg, so that several types of segmentation 

 are known. 



Cleavage in Homolecithal Eggs. — In homolecithal eggs the early 

 cleavage is very regular. The spindle of the first mitosis is horizontal 

 (assuming the animal pole to be above) and near the center of the egg. 

 The first cleavage plane is therefore meridional, passing through both 

 animal and vegetative poles, and dividing the fertilized egg into two 

 approximately equal cells. The sea-cucumber Synapta (Fig. 169) is a 



