32 



MOLLUSC A. 



stituents of the germinal spot has been found by Flemmiog in 

 Tichogonia, in which however the more highly refracting body 

 envelopes part of the less highly refracting 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 spermatozoa 

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

 becoming ova and others 



spermatozoa. The ova are R 



usually formed in the wall 

 of the pouch, and the sper- 

 matozoa internally (Pul- 

 monata) (fig. 13 A), or a 

 further differentiation of 

 parts may take place (fig. 

 13 B}. The ova of Gastero- 

 pods are exceptional in the 

 fact that a vitelline mem- 

 brane is rarely or never 

 developed around them. 

 The ovum in its passage 

 to the exterior becomes en- 

 closed 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 de- 

 veloped on the wall of the follicle, and the seminal 

 masses (b) internally. 



B. Of Aeolidia. The seminal portion of a 

 follicle is beset peiipherally 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 penetrates 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 completely 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 



