28 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



tebrates it has been shown that the number of chromosomes in the oogonia is even, 

 the number in the spermatogonia odd, and that all the mature ova and half the 

 spermatids contain an extra or accessory chromosome (see p. 32). 



Previous to fertilization, the ova undergo a similar process of maturation. 

 Two cell divisions take place but with this difference, that the cleavage is un- 

 equal 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 body 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 

 caUed 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. 

 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 

 (see diagram B, Fig. 12). Each spermatid, however, may form a mature sper- 

 matozoon, but only one of the four daughter cells of the primary oocyte becomes a~~ 

 mature ovum. The three polocytes are abortive and degenerate eventually, 

 though it has been shown that in the ova of some insects the polar body may be 

 fertilized and segment several times like a normal ovum. The maturation of 

 human ova has not been observed, but such a process probably takes place. The 

 reduction of the chromosomes may be best observed in the ova of Ascaris and of 

 insects. The mouse offers a favorable opportunity for studying the maturation 

 of a mammalian egg as the ova are easily obtained. Their maturation stages 

 have recently been studied by Mark and Long (Carnegie Inst. Publ. No. 142). 



Maturation of the Mouse Ovum. — The nucleus of the mature ovum 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 of 

 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 follicle. In the forma- 

 tion of the maturation spindle no astral rays and no typical centrosomes have 

 been observed. The chromosomes are V-shaped. The first polar body is seg- 

 mented from the ovum and lies beneath the zona pellucida as a spherical mass 

 about 25 micra in diameter (Fig. 13). Both ovum and polar body (secondary 

 oocytes) contain 10 or 12 chromosomes, or half the number normal for the mouse. 



