1 86 ZOOLOGY FOR MEDICAL STUDENTS chap. 



is quite tiny — forming the second polar body (Fig. 84, C, II) — while the 

 large cell is now the macrogamete or mature egg. 



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

 i.e. the first polar body, also divides with mitosis into two daughter cells 

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

 Its exceptional occurrence is however of importance for in such a case 

 we see clearly the fundamental identity of the processes at work in the 

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

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

 two mitotic 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. 



The conspicuous difference between the two sexes is a comparatively 

 superficial 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, functionless 

 polar bodies. 



Here we have come in touch with the most characteristic difference 

 between the gametes of the two sexes throughout the animal kingdom. 

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

 store 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 

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

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

 the external genital opening.^ A single microgamete. attaches itself 

 by its broad end to an egg (Fig. 85, A) and the nuclear material 

 from it passes, together with the centrosome, into the cytoplasm of 

 the egg. The two nuclei which now are in the cytoplasm — the 

 original egg nucleus (N) and the immigrant sperm nucleus (w) — 

 undergo a gradual increase in size, the two chromosomes in each 

 becoming lengthened out into slender meandering filaments. Eventually 



1 For the sake of clearness we describe the processes of maturation and, 

 syngamy in their logical sequence but as a matter of fact in Ascaris the two 

 processes overlap, the formation of the polar bodies being delayed until after 

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



