MOLLUSCA. 



rarely or never developed around them. The ovum in its pas- 

 sage to the exterior becomes enclosed in a secretion of the 

 albuminous gland, which hardens externally to form a special 

 membrane. 



FIG. 13. FOLLICLES OF THE HERMAPHRODITE GLANDS OF GASTEROPODA. 



(From Gegenbaur.) 



A. Of Helix hortensis. The ova (aa) are developed on the wall of the follicle, 

 and the seminal masses (/>) internally. 



B. Of Aeolidia. The seminal portion of a follicle is beset peripherally by ovarian 

 saccules (a), c. Common afferent duct. 



Cephalopoda. 



Lankester (No. 32) has brought out some very interesting 

 points with reference to the nutrition of the eggs of Sepia during 

 their growth. The eggs develope in connective-tissue pouches 

 which early give rise to a double pedunculated capsule of 

 connective tissue. The cells of the inner layer of this capsule 

 soon assume an epithelial character, and become a definite 

 follicular epithelium, while between the two layers there pene- 

 trates a network of vascular channels. The follicular epithelium 

 becomes after the establishment of these vascular channels 

 folded in a most remarkable manner. The folds, which are 

 shewn in section in fig. 14, ic, project into and nearly com- 

 pletely fill up the body of the ovum. An enormous increase is 

 thus effected in the nutritive surface exposed by the epithelium. 

 Each fold is thoroughly supplied with blood-vessels. The 

 plications of the follicular epithelium give rise to a basket-work 

 tracery on the surface of the ovum. During the stage when the 

 follicular epithelium has the above structure, its cells have a 



