THE OVUM. 39 



The eggs of the Lamellibranchiata are not only remarkable 

 in the possession of a micropyle, but in certain peculiarities of the 

 yolk and of the germinal vesicle. 



In the fresh-water mussels there is usally found in young and 

 medium-sized ova a peculiar lens-shaped body Keber's cor- 

 puscle which is placed immediately internal to the micropyle. 

 It is probably in some way connected with the nutrition of the 

 ovum, though the fact that it is not always present shews that it 

 cannot be of great importance. 



A dark body found by von Jhering in the neighbourhood of 

 the germinal vesicle in the ripe ovum of Scrobicularia is probably 

 of a similar nature to Keber's corpuscle. Both bodies may be 

 placed in the same category as the so-called yolk nucleus of the 

 spider's and frog's ova. 



In all except the youngest ova of Anodon and Unio the 

 germinal spot is composed of two nearly complete spheres united 

 together for a small part of their circumference. (Fig. 12, gs.} 

 The smaller of these has a higher refractive index than the 

 larger, and often contains a vacuole : the two parts together 

 appear to be the separated components (though not by simple 

 division) of the primitive nucleolus. A nucleolus of this charac- 

 ter is not universal amongst Lamellibranchiata, but a similar 

 separation of the constituents of the germinal spot has been 

 found by Flemming in Tichogonia, in which however the more 

 highly refracting body envelopes part of the less highly re- 

 fracting body in a cap- like fashion. 



Gasteropoda. 



The ova of the Gasteropoda are developed, like those of the 

 Lamellibranchiata, from the epithelial cells of the ovarian acini 

 or pouches. In the hermaphrodite forms both ova and sperma- 

 tozoa are produced in the same pouches (fig. 13), some of the 

 epithelial cells becoming ova and others spermatozoa. The ova 

 are usually formed in the wall of the pouch, and the sperma- 

 tozoa internally (Pulmonata) (fig. 13 A], or a further differenti- 

 ation of parts may take place (fig. 13 B}. The ova of Gastero- 

 pods are exceptional in the fact that a vitelline membrane is 



