204 THE BEGINNINGS OF LIFE. 



Martin Barry and also Allen Thomson incline to the 

 latter view, whilst Bischoff, Valentin, and others, 

 maintain that the germinal vesicle of the ovum first 

 appears within the ovisac, and that the latter is there- 

 fore the primary formation. It is stated, however, 

 both by Dr. Martin Barry and Dr. Thomson, that the 

 ovisac (if it does precede) could only be formed c a very 

 short period' before the rudiments of the ovum, because 

 even where it is most minute it is found to co-exist 

 with the germinal vesicle. And at this early stage (as 

 they and all others admit) there is only a trace of the 

 future yolk, and none of the cells which subsequently 

 compose the membrana granulosa. The development of 

 these cells, at all events, and the further development 

 of the ovum, undoubtedly take place within the ovisac. 



ovum appears as an almost free anatomical element, situated, at a 

 certain portion of its circumference, in the midst of a granular and rudi- 

 mentary cell structure. At its period of maturation, the Graafian vesicle 

 bursts and sets free the contained ovum. \Ve extract the following 

 from Dr. Thomson's article : ' In the human ovary these follicles are 

 firm spheroidal sacs which attain when mature an average size of about 

 jj of an inch. In the ovaries of women during the child-bearing period, 

 a number of smaller follicles lie throughout the greater part of the sub- 

 stance of the ovary ; the more developed follicles being usually placed 

 towards the free surface, but at some little distance from it. As they 

 enlarge and approach maturity, the ovarian substance seems to give way 

 to them, or to become gradually thinner between the follicles and the 

 outer surface of the ovary, so as at last to leave almost nothing but the 

 covering membranes of the ovary at the most projecting part. . . . 

 The following are the results of a few measurements made by myself 

 and others of the external diameter of the mature ovarian ovum, viz. 

 man ^ dog T ^, cat ^ rabbit -^ rat ^, mouse ^, pig 

 cow 25^, guinea-pig -^ of an inch.' (Loc. cit. pp. 81-83.) 



