38 MOLLUSCA. 



ovum becomes ripe a large space filled with albuminous fluid 

 becomes established between the ovum and its membrane, but 

 the ovum remains attached to the membrane at the micropyle. 

 In Scrobicularia (von Jhering, No. 25) the membrane round the 

 ovum appears from the first as an albuminous layer, the outer- 

 most stratum of which becomes subsequently hardened as the 

 vitelline membrane. In this form also the protoplasmic stalk 

 becomes, in pouches largely filled with ova, extremely long. 

 The ova become eventually detached by the stalk rupturing, 

 and the portion of it which remains attached to the vitelline 

 membrane falling off". The function of the stalk and of the 

 micropyle during the development of the ovum is undoubtedly a 

 nutritive one. 



In Anodon and Unio yolk granules 

 similar to those deposited in the proto- 

 plasm of the ovum are also found in the 

 epithelial cells of the ovarian pouches 

 (Flemming, 22), and there can be but 

 little doubt that they are directly trans- 

 ported from these cells into the ovum. 

 These cells would seem therefore to play 

 much the same part as the yolk-glands 

 of some Turbellarians (Prostomum cale- 

 donicum). In Scrobicularia yolk granules 

 are not found in the epithelium of the PLAN ATA. (After Flemming.) 

 pouches, but are contained in the dilated m in?Upt. Cr Py 

 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 Anodon 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 Valparaizo, it was equal to nearly the circum- 

 ference of the ovum. 



