36 



THE UNIMPREGNATED OVUM. 



In the first stages of the development of its internal structure, the 

 ovary, according to Valentin,* closely resembles the testis. A number of 

 streaks are first seen running from the surface of the organ towards the 

 solid axis. These streaks become tubes closed at either extremity, and 

 having membranous walls lined with epithelial globules. In the cavity 

 of these tubes, which are distinctly visible in the embryos of sheep or 

 cows from three to five inches in length, the ovarian follicles are soon 

 developed- in the form of cellules, with transparent walls and granular 

 contents, arranged in a linear manner. In proportion as the follicles 

 increase in number and size, the walls of the tubes in which they are con- 

 tained become thinned, and the central solid axis of the ovary relatively 

 smaller. At length the tubes become so pressed together and displaced by 

 the enlargement of the follicles within them, that the tubular structure is 

 scarcely recognisable. Nevertheless, with some patience the tubes may be 

 distinguished and even demonstrated separately in the young calf, sheep, 

 cat, and rabbit, at the time of birth. While the Graafian follicles enlarge, 

 their contents become more fluid. The fluid part collects in the middle 

 of the follicle, while the granules which from the first have a linear 

 arrangement form an investment, the membrana granulosa, on the inner 

 surface of the follicle. The order in which the different parts of the 

 ovum are developed, Valentin could not ascertain. 



These observations of Valentin, though detailed as if they were made 

 with great care, have not been confirmed by either Barry or Bischoff.t 



Dr. Barry,]; it is true, does not appear to have examined the state of 

 the ovary in foetal animals. It was in young animals, and in those which 

 had just reached puberty that he studied the subject. 



The results at which he arrived were that there is a continual disap- 

 pearance of ova and formation of new ones from a very early age, that 

 myriads of ovisacs with their contents are formed which never reach matu- 

 rity, and that the stroma of the ovary always contains innumerable groups 

 of these immature ovisacs. The part first formed, however, according to 

 Dr. Barry, is not the ovisac but the germinal vesicle. This becomes sur- 

 rounded by a coating of oil-like globules and peculiar granules, and subse- 

 quently by a membrane which is the ovisac or Graafian follicle. The 

 ovisac, as yet transparent and structureless, enlarges, and the granules 

 within increase in number. Next a clear space forms around the ger- 

 minal vesicle, which occupies the centre of the ovisac. In this space the 

 oil-like globules accumulate, and minute opaque granules show themselves 

 amongst them. Thus is formed the yolk, which next becomes separated 

 from the granules in the general cavity of the ovisac by the development 



Mullet's Archiv. 1838, p. 526, et seq. 



t Bischoff expressly states that he has never seen the streaks and tubes described by 

 Valentin, though he sought them in the embryoes of several mammiferous animals, of different 

 ages. t Philosoph. Transact., 1838. 



