186 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



my forming the second polar body (Fig. 84, C, II) while the 



.v the macrogamete or mature egg. 



Occasionally the smaller of the two cells formed by the first division, 

 !ar body, also divides with mitosis into two daughter cells 

 containing two monads but as a rule this division is suppressed. 

 :umal occurrence is however of importance for in such a case 

 e clearly the fundamental identity of the processes at work in the 

 mali- and female gonad. In each case the cell in which the reduced 

 number of (tetrad) chromosomes makes its appearance gives rise by 

 nitotic divisions in rapid succession to a set of four cells each con- 

 taining the reduced number of chromosomes monad in character. These 

 two divisions, associated with the reduction in the number of chromosomes, 

 are known as the first and second meiotic or maturation divisions. 



Tin- conspicuous difference between the two sexes is a comparatively 



rhcial one, namely that in the male each one of the four cells resulting 



from the meiotic divisions becomes a functional gamete, while in the 



female only one does so, the other three being the reduced, functioniess 



polar bodies. 



Here \ve have come in touch with the most characteristic difference 

 the gametes of the two sexes throughout the animal kingdom. 

 The female gamete is relatively large in size, frequently containing a 

 of reserve food-material or yolk, and is incapable of active move- 

 ment : whereas the male gamete is relatively small, without stored 

 food-material, and active in its movements. The fact that three out of 

 each four potential macrogametes degenerate is no doubt an adaptive 

 arrangement facilitating the increase in size of the fourth, necessary to 

 enable it to contain a sufficient store of yolk. 



The act of syngamy between the two gametes, the " fertilization of 



to use the older name, takes place in the cavity of the uterus, 



which a supply of microgametes has been passed by the male through 



the external genital opening. 1 A single microgamete attaches itself 



Toad end to an egg (Fig. 85, A) and the nuclear material 



s, together with the centrosome, into the cytoplasm of 



egg. Tin- two nuclei which now are in the cytoplasm the 



i< lens (N) and the immigrant sperm nucleus () 



undergo a -ra.lual increase in size, the two chromosomes in each 



becoming lengthened out into slender meandering filaments. Eventually 



ss we describe the processes of maturation and 

 1 sequence but as a matter of fact in Ascaris the two 

 lap, tin- formation of the polar bodies being delayed, until after 

 entered the egg (cf. Fig. 85, A). 



