504 EVOLUTION, HEREDITY, EUGENICS 



page 500) at the end of which they are transformed into Jirst spermato- 

 cytes. At this time each cell increases in size and like or homologous 

 chromosomes, which are already split longitudinally, become asso- 

 ciated in pairs (pseudo-reduction), the usual process being a side-by- 

 side synapsis, forming a quadruple chromosome known as a tetrad. 

 Two of the parts are maternal and the other two, paternal in origin. 

 Any two parts are known as dyads. 



Two maturation divisions follow in rapid succession without 

 further splitting of the chromosomes, and separate the four parts 

 of each tetrad into different cells. The two maturation divisions 

 acting as a unit separate the parts of a chromosome that were 

 derived from one parent from those that were derived from the 

 other. This reduction (segregation) may occur at either maturation 

 division. If a tetrad chromosome divides along the lengthwise 

 split (equationally) at tho. first maturation division, then each dyad 

 is one-half paternal and one-half maternal in origin. For such 

 dyads, the second maturation division segregates the maternal part 

 from the paternal. 



On the other hand, those tetrads which separated at the first 

 maturation division along the line of synapsis give one dyad of 

 paternal and one dyad of maternal origin. That is, segregation for 

 such a pair occurs at the first maturation division, and the second 

 division is consequently equational or longitudinal.^ In either case 

 the four resultant cells are called spermatids, each containing one- 

 half the number of chromosomes found in the original spermato- 

 gonium. But while a complete haploid series is maintained, the 

 spermatids differ as to the parental source of the various chromo- 

 somes, now monads, the assortment being a random one. Such a 

 process of segregation (reduction) is characteristic of the maturation 

 divisions. 



From the spermatids develop the mature spermatozoa, each 

 with a head which contains the chromosomes, a body, and a slender 

 tail, used in locomotion. The question of accessory, " sex deter- 

 mining " chromosomes will be discussed on page c^^Z- 



Oogenesis. — As in the development of the male germ cells, we 

 find that the oogonia undergo a period of multiplication, dividing 

 by ordinary mitosis (page 500). 



At the end of their preliminary mitoses, the oogonia are called 

 primary oocytes. They rapidly increase in size and their longi- 



^ Both divisions are longitudinal. 



