GENERATION. 129 



cropyle,* as in Anodonta (mussel). Segmentation partial. 

 Superficial layer not participating in the process. The cili- 

 ated and revolving embryo divides, and is crowned with a 

 valvular appendage developed from the unsegmented super- 

 ficies. The embryo divides into halves, each half being cov- 

 ered by a valve. In the middle of the angle formed by these 

 valves is raised a short hollow cylinder the byssus-forming 

 organ and out of which projects a long transparent byssus. 

 (Siebold.) 



Gasteropoda. Segmentation of vitellus complete, except- 

 ing a few hyaline corpuscles. Embryo oblong, indented, and 

 ciliated at one extremity. The indentation deepens in Eolis 

 and Doris, forming two lobules upon whose borders very long 

 cilia are developed. The crests are absent in certain pul- 

 monates. A third eminence is developed between these two 

 pinions, and ultimately changed into a foot. A spiral shell 

 and operculum are very constantly present, whether the ma- 

 ture form be conchiferous or not. Subsequently the pinions 

 disappear, and in the naked species the shell and operculum 

 are lost. From an early time in development the embryo ro- 

 tates upon its axis by alternate revolutions. In naked pul- 

 monates a sac analogous to umbilical vesicle is formed at 

 caudal region of embryo, which, during development, altern- 

 ately contracts with embryo. 



Cephalopoda. Segmentation partial. In the germinative 

 area upon the upper portion of the egg a two-layered blasto- 

 dermic membrane originates, from the central and outer 

 portion of which a disciform embryo arises. Various irreg- 

 ularities mark its surface. An irregular median fold repre- 

 sents the mantle enclosing the future visceral cavity. Upon 

 either side of it are seen the gills, the halves of the funnel, 

 cephalic lobes, and the rudimentary arms and eyes. The 

 mouth is not situated between these organs, but at the pos- 



* The micropyle is the remains of the pedicle which, in the development 

 of egg, connects it with the ovary. It is believed to be the point at which 

 entrance is effected by the spermatic filament, and is closed prior to act of 

 conception. 



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