DEVELOPMENT OF THE OVA. 



8 5 I 



fimbriated extremity of the Fallopian tube embracing the ovary, so that the ovum 

 is shed into the Fallopian tube itself.] Only a small number of the Graafian 

 follicles undergo development normally, by far the greatest number atrophy and 

 never ripen. (The study of the development of the ova and ovary was advanced 

 particularly by Martin Barry, Pfliiger, Billroth, Schron, His, Waldeyer, Kolliker, 

 Koster, Lindgren, Schulin, Foulis, Balfour, and others.) 



According to Waldeyer, the mammalian ovum is not a simple cell, but a compound structure. 

 The original primitive ovum is, according to him, formed only of the germinal vesicle and 

 germinal spot, with the surrounding membranous clear part of the vitellus (fig. 641, III). The 

 remainder of the vitellus is developed by the transformation of granulosa cells, which also form 

 the zona pellucida. 



Holoblastic and Meroblastic Ova. The ova of frogs and cyclostomata have the same type as 

 mammalian ova ; they are called holoblastic ova, because all their contents go to form cells 

 which take part in the formation of the embryo. In contrast with these, the birds, the mono- 

 tremes alone amongst the mammals {Caldwell), the reptiles and the other fishes have meroblastic 



Fig. 641. 



I, An ovarian tube in process of development (new-born girl), a, a, young ova between the 



epithelial cells on the surface of the ovary; b, the ovarian tube with ova and epithelial 



cells ; c, a small follicle cut off and enclosing an ovum. II, Open ovarian tube from a 



bitch. Ill, Isolated primordial ovum (human). IV, Older follicle with two ova (0, 0) 



and the tunica granulosa (g) of a bitch. V, Part of the surface of a ripe ovum of a rabbit 



z, zona pellucida ; d, vitellus ; e, adherent cells of the membrana granulosa. VI, First 



polar globule formed. VII, Formation of the second polar globule (Fol). 



ova (Reichert). The latter, in addition to the white or formative yelk, which corresponds to 



the yelk of the holoblastic eggs, and gives rise to the embryonic cells, contains the food-yelk 



(yeliow in birds), which during development is a reserve store of food for the developing 



61 Hen's Egg. The small, white, round, finelv granular speck, the cicatricula, blastoderm, 

 or tread, which is 2 '5-3-5 mm. broad and 0-28-0 '37 thick, lying upon the surface of the yellow 

 yelk, corresponds to the contents of the mammalian ovum, and is, therefore, the formative yelk. 

 In the cicatricula lie the germinal vesicle and spot (fig. 642). From the tread in which he the 



