THE OVUM. 



31 



mp 



FIG. 12. MEDIUM-SIZED 

 OVUM OF ANODONTA COMPLA- 

 NATA. ( After Flemming. ) 



mp. micropyle. y s. ger- 

 minal spot. 



In Anodon and Unio yolk granules simi- 

 lar to those deposited in the protoplasm of the 

 ovum are also found in the epithelial cells of 

 the ov.irian pouches (Flemming 22), and there 

 can be but little doubt that they are directly 

 transported from these cells into the ovum. 

 These cells would seem therefore to play much 

 the same part as the yolk-glands of some Tur- 

 bellarians (Prostomum caledonicum). In 

 Scrobicularia yolk granules are not found 

 iu the epithelium of the pouches, but are 

 contained in the dilated disc by which the 

 ovum is attached to the wall of its pouch, as 

 well as in the ovum itself. 



On the ovum becoming detached the 

 micropyle still remains as an aperture, which 

 probably has the function of admitting the spermatozoa. 



The shape and form of the micropyle vary greatly. In Auodon and 

 Unio it is a projecting trumpet-shaped structure, which after fertilization 

 becomes shortened and reduced to a mere aperture which is finally 

 stopped up. (Fig. 12.) 



In other forms it is simply a perforation in the vitelline membrane 

 which is sometimes very large. In a species of Area, which I had an 

 opportunity of observing at Valparaiso, it was equal to iiearly the circum- 

 ference of the ovum. 



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 usually found in young and 

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

 winch is placed immediately internal to the micropyle. It is pro- 

 bably 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, g.s.} 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 primi- 

 tive nucleolus. A nucleolus of this character is not universal 

 amongst Lamellibranchiata, but a similar separation of the con- 



