212 PRINCIPLES OF AXfMA'. HIOIJX.'Y 



always the same in the same species. Jn tlie illustration (Fig. 164), 

 they are represented as having been so placed that the maternal half 

 of the tetrad is separated from the paternal half. It is a matter of chance, 

 however, whether the paternal half is turned toward one end of the spin- 

 dle or toward the other. It may happen, therefore, that all of the pa- 

 ternal dyads go into one cell and all of the maternal dyads into the other, 

 or, as in the figure, part into one cell and part into the other. The cells 

 produced by this division are called secondary spermatocytes. 



It is worthy of note that in the division just described, no chromosomes 

 have divided. The tetrads have divided, but merely by the separation 

 of the two chromosomes which had previously come together. Such a 

 division is called a reduction division; it never occurs in cell divisions 

 except in maturation, and in only one of the maturation divisions. 



Second Maturation Division. — The secondary spermatocytes divide 

 verj' shortly by a mitosis in which the dyads are separated into two com- 

 ponents. The resulting cells are called spermatids. A given spermatid 

 may contain only paternal chromosomes, or only maternal, or both 

 paternal and maternal in any proportion. The number of these chromo- 

 somes is only half that of the original spermatogonium. 



By a transformation in shape, the spermatid becomes a mature sper- 

 matozoon. This cell consists usually of a head, which may be rounded 

 or long and slender, and a whip-like tail. The chromosomes are all con- 

 tained in the head, the tail being merely a motile organ. 



Oogenesis — The maturation of the female germ cells is in most re- 

 spects similar to that of the male. The early germ cells undergo a period 

 of multiplication in which they divide by ordinary mitosis. During this 

 time they are called oogonia (singular, oogonium) . Eventually this ordi- 

 nary division ceases, and the ceils are ready to initiate the maturation 

 process. At this time they are known as primary oocytes. These 

 oocytes grow rapidly to many times their original volume, the growth 

 being much greater than in the male. 



Tetrads. — During growth the chromosomes meet in pairs, each pair, 

 as in the male, being composed of one maternal and one paternal chromo- 

 some. Each chromosome divides as they come together, so that each 

 pair presents a quadruple body, the tetrad. These tetrads are divided 

 in the two maturation divisions, first into dyads, next into their single 

 components, in a manner strictly comparable to the divisions in the male. 



First Maturation Division. — When a spindle is formed for the first 

 division, it appears, not in the center, but near the surface; and it is 

 placed approximately perpendicular to the surface. The tetrads take 

 their place on this spindle. If the animal is one in which, as illustrated 

 in Fig. 164, the first division is the reduction division, the tetrads are 

 placed so that the paternal half is toward one end of the spindle, the 

 maternal half toward the other. The resulting cells therefore contain 



