GENERAL EMBRYOLOGY 



1605 



Primordial Germ-ccfl 



OSgonia 



divides by mitosis into two cells— namely, (i) the first polar body, 

 of small size ; and {2) the secondary ooc^-te. In the second division 

 the secondary oocyte (daughter-cell) extrudes the second polar body, 

 and then it becomes a mature ovum (female pronucleus) . In other 

 words, the secondary oocv-te divides, by mitosis, into two cells— 

 namely, (i) the second polar 

 body, of small size ; and (2) the 

 mature o\Tim (female pro- 

 nucleus), which latter only 

 undergoes further division if 

 fertilized. Prior to its matura- 

 tion, the ovum has passed 

 through three stages — namely, 

 (i) oogonium, (2) primary 

 oocyte, and (3) secondary 

 ooc\i:e. The mature ovum cor- 

 responds to a male spermatid, 

 the difference, in the case of the 

 latter, being that the spermatid 

 undergoes further transforma- 

 tion into a spermatozoon. 



In all cases the cell-divisions 

 are of the nature of a some- 

 what modified mitosis or karyo- 

 kinesis, consisting in a reduc- 

 tion-division — that is to say, a 

 reduction in the number of 

 at each cell- 



2nd Polar Body 



ist Polar Body 

 Divisions of ist 

 Polar Body 



Fig. 661. — Diagram SHOw^NG 

 Oogenesis (after Boveri). 



primary ooc>i:e 



chromosomes 

 division. 



From one 

 (mother-cell) there thus finally result four grand-daughter cells, 

 one large and three small — namely, the mature ovum (female pro- 

 nucleus) of large size, and three small polar bodies, the first polar 

 body, as a rule, having divided into two small cells. The mature 

 o\-um is capable of fertilization, but the polar bodies (abortive 

 ova) are inactive and disappear. 



Table of Comparison between the Hale and Female Germ-cells. 



Male. 



Spermatogonium 

 Primary spermatocyte . . 

 Secondary spermatocyte 

 Spermatid 



Female. 



Oogonium. 

 Primary oocyte. 

 Secondary oocj'te. 

 Mature ovum. 



Though there is a great resemblance between spermatogenesis 

 and oogenesis, two differences are to be noted: (i) The final result 

 in oogenesis is the formation of four cells — namely, {a) the mature 

 oxTim, of large size, and capable of fertilization; and (6) three, as 

 a rule, polar bodies, all small, quite inactive, and subsequentty 



