THE OVUM. 



FIG. 7. SECTION 

 THROUGH A SMALL 

 PART OF THE SURFACE 

 OF AN OVUM OF AN 



It is in many cases doubtful whether a par- 

 ticular membrane is a chorion or a vitelline 

 membrane. 



All the membranes which surround the 

 ovum may be provided with a special aper- 

 ture known as the micropyle. A micropyle 

 is by no means found in the majority of 

 types, and there is no homology between 

 the various apertures so named. Micropyles 

 have two functions, either (i) to assist in the 

 nutrition of the ovum during its develop- 

 ment, or (2) to permit the entrance of the 

 spermatozoa. The two functions may in 

 some cases coexist. Micropyles of the first 

 class are developed at the point of attach- 

 ment of the ovum to the wall of the ovary or to its follicle. 

 Good examples of this kind of micropyle are afforded by the 

 Lamellibranchiata (fig. 12), Holothuria, and many Annelida 

 (Polynoe, etc.). The micropyle of the Lamellibranchiata (p. 37) 

 probably serves also to admit the spermatozoa. The second 

 type of micropyle is found in many Insecta, Teleostei, etc. 



fe. Follicular epi- 

 thelium, vt. Vitelline 

 membrane. Zn. Zona 

 radiata. yk. Yolk with 

 protoplasmic network. 



GENERAL BIBLIOGRAPHY OF THE OVUM. 



(1) Ed. van Ben ed en. " Recherches sur la composition et la signification de 

 1'ceuf," etc. Mem. cour. d. VAcad. roy. des Sciences de Belgique, Vol. xxxiv. 1870. 



(2) R. Leuckart. Artikel ' Zeugung," R. Wagner's Handworterbuch d. Physio- 

 logie, Vol. iv. 1853. 



(3) Fr. Leydig. " Die Dotterfurchung nach ihrem Vorkommen in d. Thierwelt 

 u. n. ihrer Bedeutung." Oken. Isis, 1848. 



(4) Ludwig. " Ueber d. Eibildung im Thierreiche." Arbeiten a. d. zooL-zoot. 

 Instilut Wiirzburg, Vol. i. I874 1 . 



(5) Allen Thomson. Article " Ovum" in Todd's Cyclopaedia of Anatomy and 

 Physiology, Vol. v. 1859. 



(6) W. Waldeyer. Eierstock u. Ei. Leipzig, 1870. 



1 A very complete and critical account of the literature is contained in this paper. 



