THE OVUM. 



MOLLUSCA. 

 Lamellibranchiata. 



(21) H. Lacaze-Duthiers. "Organes genitaux des Acephales Lamelli- 

 branches." Ann. Sci. Nat., 4 me serie, Vol. II. 1854. 



(22) W. Flemming. " Ueb. d. er. Entwick. am Ei d. Teichmuschel. " Archiv 

 f. mikr. Anat., Vol. x. 1874. 



(23) W. Flemming. " Studien lib. d. Entwick. d. Najaden." Sitz. d. k. 

 Akad. Wiss. Wien, Vol. LXXI. 1875. 



(24) Th. von Hessling. "Einige Bemerkungen, etc." Zeit. f. wiss. Zool., 

 Bd. v. 1854. 



(25) H. von Jhering. " Zur Kenntniss d. Eibildung bei d. Muscheln." Zeit. 

 f. wiss. ZooL> Vol. xxix. 1877. 



(26) Keber. De Introitu Spermatozoorum in ovuta, etc. Konigsberg, 1853. 



(27) Fr. Leydig. " Kleinere Mittheilung etc." Mullet's Archiv, 1854. 



Gasteropoda. 



(28) C. Semper. " Beitrage z. Anat. u. Physiol. d. Pulmonaten." Zeit. f. 

 wiss. Zool., Vol. vni. 1857. 



(29) H. Eisig. "Beitrage z. Anat. u. Entwick. d. Pulmonaten." Zeit. f. wiss. 

 Zool., Vol. xix. 1869. 



(30) Fr. Leydig. " Ueb. Paludina vivipara." Zeit.f. wiss. Zool., Vol. II. 1850. 



Cephalopoda. 



(31) Al. Kolliker. Entwicklungsgeschichte d, Cephalopoden. Zurich, 1844. 



(32) E. R. Lankester. "On the developmental History of the Mollusca." 

 Phil. Trans., 1875. 



L amellibranchiata. 



The ova of the Lamellibranchiata present several points of 

 interest. They are developed in pouches of the ovary which are 

 lined by a flattened germinal epithelium, or sometimes (?) a 

 syncytium. Some of the cells of this epithelium enlarge and 

 become ova, but remain attached to the walls of their pouches 

 by protoplasmic stalks. Round the ovum there appears in some 

 forms (Anodon, Unio) a delicate vitelline membrane, which is 

 incomplete at the protoplasmic stalk, and is therefore perforated 

 by an aperture which forms the micropyle. (Fig. 12.) As the 



