ORGANIC CHEMISTRY AND CELL DIVISION 



105 



nucleus. There is often a special aperture in the wall of the egg called 

 a micropyle ( ) through which the sperm enters. 



Usually only one sperm cell enters an egg. Various changes are set up 

 at the very moment the sperm enters the egg causing the egg membrane 

 to become impervious to other sperm, though sometimes, if the egg be 

 old or diseased, this process may not begin soon enough, so that several 

 sperm enter the same egg. This is called multiple fertilization. There 



O 



A, one-celled stage B, two-celled stage C. four-celled stage D. eight-celled stag* 



//, many-celled 

 stage 



Fig. 34. Cleavage of Frog's Egg. 



are some species in which this multiple fertilization occurs normally. 

 Monstrosities are often formed in this way. 



When the two pronuclei unite they form a fusion nucleus (Fig. 32), 

 also called the first segmentation nucleus. The egg is then said to be 

 fertilized, or impregnated. 



The full quantity of chromosomes is now again present and there 

 seems to be an impulse brought with them which starts the egg dividing. 

 This division of the fertilized egg is known as segmentation or cleavage 

 (Fig. 34). This is brought about by ordinary mitosis, and these first 

 cells which come into being by the splitting of the fertilized cell are 

 called blastomeres ( ). The chromosomes do not 



divide longitudinally in these blastomeres but each new cell receives 

 one-half of the material brought by each of the parent cells. In this 

 way every cell in the body gets an equal amount of chromosome mate- 

 rial from each of its parents. And in this way also, every cell in the 

 body of an individual has exactly the same number of chromosomes 

 within it that every other cell has. 



Each succeeding division of cells produces cells a trifle smaller than 

 the parent cell. 



The cells divide differently with different quantities of yolk. Usu- 

 ally the first thre cleavage planes are perpendicular to each other. If 

 the yolk is evenly distributed the newly formed cells will be more or 

 less of equal size. 



Often the yolk collects at the lower portion of the egg. This is 

 undoubtedly due to the force of gravity. In such cases the protoplasm 

 gathers at the upper end. The upper end is then called the active, 

 formative, or animal pole and the lower the passive, nutritive, or veg- 

 etable pole. The polar bodies are usually freed at the formative pole. 



