1 8 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



cell divisions take place but with this difference, that the cleavage is unequal, 

 and, instead of four cells of equal size resulting, there are formed one large ripe 

 ovum or oocyte and three rudimentary or abortive ova known as polar bodies 

 or polocytes. The number of chromosomes is reduced in the same manner as in 

 the spermatocyte, so that the ripe ovum and each polar cell contain one-half 

 the number of chromosomes found in the immature ovum or primary oocyte. 

 The female germ cells, from which new ova are produced by cell division, are 

 called oogonia and their daughter cells after a period of growth within the ovary 

 are the primary oocytes, comparable to the primary spermatocytes of the male 

 (Fig. 12). During maturation the ovum and first polocyte are termed secondary 

 oocytes (comparable to secondary spermatocytes), the mature ovum and second 

 polocyte, with the daughter cells of the first polocyte, are comparable to the 

 spermatids. Each spermatid, however, may form a mature spermatozoon, but 

 only one of the four daughter cells of the primary oocyte becomes a mature ovum. 

 The ovum develops at the expense of the three polocytes which are abortive and 

 degenerate eventually, though it has been shown that in the ova of some insects 

 the polar cell may be fertihzed and segment several times hke a normal ovum. 

 In most animals, the actual division of the first polocyte into two daughter cells 

 is suppressed. The maturation of human ova has not been observed, but such a 

 process undoubtedly takes place. The reduction of the chromosomes may be 

 best observed in the germ cells of Ascaris and of insects. The mouse offers a 

 favorable opportunity for studying the maturation of a mammahan egg as the ova 

 are easily obtained. Their maturation stages have recently been studied by Long 

 and Mark (Carnegie Inst. Publ. No. 142). 



Maturation of the Mouse Ovum. — The nucleus of the ovum after maturation 

 is known as the female pronucleus. When the spermatozoon penetrates the mature 

 ovum it loses its tail, and its head becomes the male pronucleus. The aim and 

 end ol fertilization consists in the union of the chromatic elements contained in the 

 male and female pronuclei and the initiation of cell division. In the mouse, the 

 first polocyte is formed while the ovum is still in the Graafian folhcle. In the 

 formation of the maturation spindle no astral rays and no typical centrosomes 

 have been observed. The chromosomps are V-shaped. The first polar cell is 

 constricted from the ovum and Hes beneath the zona pellucida as a spherical mass 

 about 25 micra in diameter (Fig. 14). Both ovum and polar cell (secondary 

 oocytes) contain 20 chromosomes, or half the number normal for the mouse. 

 The first maturation division is the reductional one and the chromosomes take 

 the form of tetrads. 



