Chemistry of Livixg Matter and Cell Division 



101 



sperm\ 9 egg 



\ / 



z 



2. After a varying number of divisions, the many new cells thus 

 produced go through a process of growth. They are then called primary 

 oocytes and spermatocytes. 



3. These then "ripen" or "mature," after which fertilization can 

 take place. 



From what we shall soon learn regarding Paramoecia we know that 

 the chromosomes are the important carriers of all physical traits inher- 

 ited by a child from the parent. But, unless there is some method by 

 which the chromosomes throw off one-half their number, each child, 

 being the result of an egg and a sperm mating, would possess every- 

 thing its mother possessed, plus everything its father had. A super-race 

 would thus be produced which in a very few generations would be 

 totally unlike any of its parents. One can imagine what it would mean 

 to have every child twice as strong, and twice as tall, as its parents. 

 It would not be long before men would be thousands of feet tall, and 

 there would be little room for more than one or two people in the world. 

 But nature apparently loves an average, and so somewhere, the chromo- 

 somes are halved. 



The ripening process is known as 

 the maturation division (Fig. 30). 



The egg varies from the sperm in 

 the number of complete functioning 

 cells it produces, although the chro- 

 matin acts alike in both cases. 



From the primordial egg cell only 

 one mature egg develops, while three 

 undeveloped eggs, called polar bodies, 

 are formed. These latter degenerate 

 and have no known function. Each 

 sperm cell, however, develops into 

 four complete functional spermatozoa, 

 any one of which may fertilize an egg. 



Notwithstanding this difference, 

 both sperm and egg cell have the same 

 number of chromosomes characteristic 

 of the species. This full quota of 

 chromosomes is called the diploid 

 number. 



The primordial cells (those which 

 are to become eggs) begin their growth 

 very early in the embryo. Usually, a 

 quantity of yolk is deposited to serve 

 as food for the embryo which is in turn 

 to develop from the egg. 



The chromatin in the nucleus 



Fig. 31. 

 A. Diagram of the derivation of the sex 

 cells. z., the fertilized egg (zygote) ; 

 som., the body plasm (soma) ; t., the de- 

 velopment period during which the germ 

 plasm and the body plasm are indistinguish- 

 able; sp., spermary; ov., ovary; p., primor- 

 dial germ cells; u., the period of rapid in- 

 crease in number and diminution in size 

 (the number of divisions is much greater 

 than shown) ; v., the period of increase in 

 size with differentiation of cytoplasm; w., 

 the two maturation divisions; ph., polar 

 bodies; e., egg. (After Boveri.) 

 B. Spermatozoa of Rena esculenta. mp., 

 middle piece. 



C. Spermatozoa of Rana fusca. (After 

 Leydig.) 



