GENERAL EMBRYOLOGY 1607 



the second polar body, and one of each remains within the nucleus 

 or germinal vesicle of the ovum, which latter is no\\\mature. 



Each of the four cells which finally result from the primary o6c\i:e 

 — namely, the mature ovum and the three polar bodies — contains 

 two single chromosomes. 



The preceding remarks, as illustrating reduction-division in 

 chromosomes in the primary ooc}i;e, or o\'um prior to maturation, 

 may be taken as applicable, for the purpose of illustration, to the 

 primary spermatoc}i:e. Each of the ioui spermatids, resulting 

 from a primary spermatocyte, will therefore contain two single 

 chromosomes. 



Each of the two single chromosomes of the mature ovum and of 

 the spermatid respectively represents one-quarter of one of the two 

 tetrads present in the primary oocjrte and primary spemiatoc\i:e. 

 Moreover, the number of single chromosomes in the mature oxiim 

 and in the three polar bodies, as well as in each of the four sper- 

 matids — namely, two — ^represents the number of tetrads of chromo- 

 somes in the primary' o6c}i:e and primary spermatocyte respectively 

 — namely, two. 



Reduction-div-ision, therefore, in the sexual germ-cells consists 

 in (i) the division of tetrads into dyads, and (2) the allocation of 

 dyads to cell-divisions. 



Ovulation. 



The ovum lies for some time within a Graafian follicle. At this 

 period it is embedded within a heap of cells, kno\\Ti as the discus 

 proligerus. The innermost cells of this discus, which are in direct 

 contact with the o\'Tim, form the zona pellucida or zona radiata^ 

 and two or three layers of the succeeding cells give rise to the 

 corona radiata. Within the Graafian follicle, besides the discus 

 proligerus and o\'um, there is some fluid, called the liquor folliculi. 



Ovulation is the extrusion of the ovum from the Graafian follicle. 

 As a follicle becomes mature, it approaches the surface of the ovary, 

 and, when quite mature, it lies close beneath the surface. This part 

 of the follicle presents a slight projection, on which there is a pale 

 spot, called the stigma. The stigma, becoming very much attenu- 

 ated, ruptures. The liquor folliculi then escapes, carrvang with it 

 the ovum, surrounded by the corona radiata and the zona pellucida 

 or radiata, these, as stated, being derived from the discus proligerus. 

 The expelled ovum is, as a rule, conducted by the grooved ovarian 

 fimbria of the corpus fimbriatum to the ostium ahdominale of the 

 Fallopian tube. Here it enters that tube, and is gradually conveyed 

 into the cavity of the body of the uterus, where, if previously fer- 

 tilized, it undergoes development into the embryo, and then into 

 the foetus. 



Abnormal Conditions. — (i) The ovum may never leave the Fallopian tube, 

 and, if fertilized, it would give rise to tubal pregnancy. (2) When expelled 

 from the Graafian follicle and ovary, the o\Tim may drop into the abdominal 

 cavity, and, if fertihzed under these conditions, it would give rise to abdominal 



