OVUM. 



[95] 



Fig. 65 *. 



Formation of the ovarian ovum in the Frog. 



A, and B. Magnified representations of an ovarian 

 follicle and its contents in an early stage of the 

 formation of the ovum. The follicle is T L" in 

 diameter: in A the follicular membrane and its 

 epithelial lining are chiefly brought into focus; in 

 B the parts of the ovum within are represented 

 when the microscope was adjusted so as to place 

 them in focus. The wall of the ovarian follicle con- 

 sists of a structureless membrane or ovicapsule, and 

 an external covering of thin flattened cells; the 

 epithelial cells of the follicle within are seen in pro- 

 file towards the margin, and full towards the centre 

 (in A) where their granular contents and nuclei are 

 distinct. In the centre of B the large germinal vesi- 

 cle with numerous macula? is seen ; around it a clear 

 space which is a part of the basement substance of the 

 primitive yolk, and between this and the wall of the 

 follicle there is seen superiorly the dark granular 

 mass which has been called yolk nucleus. The clear 

 primitive yolk is also surrounded by a finely granu- 

 lar vitelline substance which has begun to be de- 

 posited. 



line of the clear part remains remarkably 

 smooth and well-defined for a time, and 

 there appears to be some fluid or different 

 substance interposed between it and the 



inner surface of the ovicapsule. The homoge- 

 neous membrane of the latter is found at an 

 earlv period to be lined by a single layer of very 

 distinct largely nucleated cells, which lie flatly 

 applied against its inner surface, but bulge or 

 project roundly on their other sides towards 

 the ovum. This layer of cells no doubt cor- 

 responds to the tunica granulosa of the ovisac 

 in other animals, and has a similar destina- 

 tion. 



There is as yet, neither in the earlier ova nor 

 in those half-grown, any zona or other proper 

 vitelline membrane; and it is obvious that what 

 some authors* have described as such could 

 be nothing more than the distinct surface of the 

 primitive yolk. Whether this surface becomes 

 condensed into a membrane, or at what time 

 this may occur, has not yet been determined 

 by observation. 



Besides these parts in the early Batrachian 

 ovum, there is another which has frequently 

 been seen by various observers from Von 

 Baer downwards, and which, as it is different 

 from anything that has been observed in the 

 ova of other Vertebrata, deserves some atten- 

 tion ; I refer to a dark mass of granules situ- 

 ated excentrically or towards the side of the 

 clear primitive yolk-substance, and between 

 it and the tunica granulosa of the ovisac, and 

 which, from its supposed connection with the 

 formation of the yolk-substance, has been 

 called the yolk-nucleus. This mass may easily 

 be seen in ovisacs of the common frog of 

 from T -^ to -J-ITJ. of an inch in diameter. It 

 is then about one-tenth of the diameter of 

 the ovisac. It is very opaque as compared 

 with the other parts, being composed of ag- 

 gregated heaps or small balls of finer granules. 

 The opaque granules of the yolk have been 

 supposed to be derived from this body, and 

 it has been alleged that, as the yolk-substance 

 increases, this yolk-nucleus gradually disap- 

 pears or spreads itself round the germinal 

 vesicle. -J- Leuckart, however, states that this 

 body is not invariably present, and that it is 

 subject to considerable varieties, and he is 

 not inclined to attribute to it any important 

 function in connection with the formation of 

 parts of the ovum. I have in general Ibund 

 it present, and think it more probable that it 

 may be destined to form the external and 

 larger corpuscles of the yolk, while the clearer 

 part immediately surrounding the germinal 

 vesicle may contribute to the production both 

 of these and of the finer substance in which 

 the germinal vesicle is found imbedded. But 

 farther observations will be required for the 

 determination of these points. 



As the growth of the ovarian ova proceeds, 

 the deposit of fine granules in and around the 

 primitive albuminous yolk-mass increases 

 rapidly ; and the yolk-nucleus, becoming less 

 distinct, finally disappears at an early but 

 somewhat variable period. The yolk-sub- 



* As Cramer, loc. cit. p. 21. 



J See V. Carus in Zeitsch. fur Wissen. Zool. vol. 

 ii. p. 103. ; and Ecker in his new edition of K. 

 Wagner's IconesPhysiolog. descript. of Tab. xxiii. 



