STRUCTURE OF THE OVUM. 177 



membranes ; the external thick one permits these characters to 

 be very distinctly seen, whilst with sufficiently powerful lenses 

 (1 x 1000) they may also be distinguished in the inner mem- 

 brane. Pfliiger (84) describes similar relations as existing in 

 the cells of the epithelium of the ovum attached to the zona 

 pellucida in Mammals. Leydig (68) has also given an account 

 of similar structural arrangements in the ova of Insects, and 

 especially in those of Timarcha tenebricosa; they have long 

 been recognized in the ova of the Holothuriadae and they con- 

 sequently appear to be of very general occurrence. 



I believe that the formation of the vitelline membrane must 

 be ascribed to a process allied to that by which the cuticular 

 membrane of the follicular epithelial cells is formed. In the 

 mammalian ovum it is effected by those cells of the discus 

 proligerus which constitute the epithelium of the ovum. The 

 mode in which it is effected is probably that a portion of the 

 protoplasm of the epithelial cells undergoes a cuticular trans- 

 formation, whilst delicate fibres of unaltered protoplasm remain 

 sticking into the metamorphosed mass. Reich ert (95), and 

 especially Pfluger (84), have already, in a similar manner, ex- 

 plained the vitelline membrane to be a product of the follicular 

 epithelium. 



Several observers, as Ransom (91) and H. Meyer (76), have described 

 a second very fine membrane within the zona pellucida, and in im- 

 mediate contact with the yolk, occurring especially in Mammals and 

 in Fish. This would constitute the vitelline membrane, in the strict 

 sense of the word. I must observe, however, that I have not 

 hitherto been successful in discovering this membrane, and Bischoff 

 (17, 18) has directly denied its existence. Besides the pores the 

 zona pellucida of many animals present a remarkably large open- 

 ing leading to the yolk, which constitutes the micropyle of Keber 

 (55). This has been very clearly demonstrated in the ova of 

 many invertebrate animals, and especially in Insects. It has also 

 been shown to exist in Fishes by Bruch (25) and Reichert (93). In 

 Insects the surface of the ovum in the immediate vicinity of the 

 micropyle often presents the most delicate sculpture, in consequence 

 of which it projects to some extent. See in particular the description 

 given by Leuckart (66). 



Kin Fishes also certain appendages may be sometimes seen arranged 

 VOL. II. N 



