52 



OBSTETRICAL ANATOMY 



^^2r^- 



are attracted to the contiguous surfaces : some to that of the ovisac, 

 which thus becomes lined by a thicker layer of ceils ; others to the 

 ovum, accumulating round it. With the enlargement of the ovisac, the 

 stroma of the ovum condenses round its delicate membrane (Fig. 32, h), 

 to form what has been called the " theca folliculi " of Baer — the fibrous 



tunic already noted ; this vascular 

 tunic (a), with the other (h), consti- 

 tuting the vesicle; while the stratum 

 of nucleate cells lining this double 

 covering is the membrana granulosa, 

 and those surrounding the ovum itself 

 form the ])roligcrous disc (e), the mass 

 of cells adhering thereto being the 

 cu7mihis. 



The hyalinion, or proper tunic of the 

 ovum, thickens into the clear substance 

 improperly named the zona pelhicida 

 ( /'), which in reality is a bag. x\s the 

 cells and cell-nuclei of the ovum become 

 developed, they are pushed deeper into 

 the stroma by those of more recent 

 formation ; while, as the ovum ripens, 

 the cells immediately around it become 

 elongated and pyriform, the tapering 

 extremity being attached to the zona : 

 those of the cumulus diverge irregularly 

 into the fluid intervening between them 

 and the membrana granulosa of the 

 ovisac. What have been termed reti- 

 nacnlce (d) have been described by some 

 authorities ; they are four processes 

 formed by the cells of the cumulus, 

 and may be merely exceptional diver- 

 gences. 



Until puberty there is no great ac- 

 tivity apparent in the vesicles ; but at 

 this time the ovary becomes more vas- 

 cular, and certain vesicles increase in 

 volume. At the period of "rut" or 

 " heat," one or more vesicles, accord- 

 ing to the species of animal, show 

 evidence of increased vascularity and 

 become distended ; the ovisac thins at 

 the most prominent part to which the 

 ovum tends, and blood is extravasated into it ; then, partly by absorp- 

 tion and partly by pressure, the coverings give way, the proligerous disc 

 and o^Tim escape outwards, and are either received into the Fallopian 

 tube for conveyance to the uterus, or, which is very rare, fall into the 

 cavity of the abdomen. 



The size of the mature ovum in the Cow is -^Ijj, Pig -^Ijj, Cat ^i^, 



Fig. 31. 



FOBMATIOX OF THE OviSAC IN THE 



Bitch's Ovaky. 



Bitch 



ie(T> 



Rabbit 



of an inch. 



After the rupture of a Graafian vesicle and the escape of the ovum, the 

 cavity of the ovisac is filled with a clot of blood, while its walls are 

 thickened and altered in colour, being in most animals partially everted 



