IX 



MOLLUSCA 



365 



according to Faussek, are of inesodermal origin. During this period 

 the whole of the ectoderm, except that covering the inner surfaces of 

 the arms, undergoes mucous degeneration i.e. the cytoplasm of its 

 cells degenerates into slime and is cast off. How the adult ectoderm 

 is regenerated Faussek was unable to determine. 



B 



vs. .. ** .- 

 >.-1-.V... : ." 



Fic. '297. Transverse sections of embryos of young cuttle-fish, illustrating two stages in 

 the development of the genital organs. (Alter Faussek. ) 



A, Section through young embryo of I.uli'jn nilijiiris, showing the first appearance of tin 1 genital <vlls 

 in tin- mesoderin. B, Section through older embryo of >v/i/n "[liriindis, showing tin' migration of these 

 cells into a fold projecting into the coelom. ///, rudiments of gills ; <ien, genital cells ; i/J\ genital fold ; 

 //. heart ; i/i.c, mantle-cavity ; p.n, posterior aorta ; jier, pericardium ; ji.x, posterior sinus ; x. -. >liell-sar. 



At the time that the external yolk-sac is absorbed, a large 

 diverticulum, the spiral caecum, grows out from the stomach ; and 

 this circumstance, together with the enlargement of the internal 

 yolk-sac, into which some of the yolk from the external sac passes, 

 is responsible for the almost entire suppression of the cavities of the 

 kidneys and genital organs which takes place at this period. Later, 

 when the yolk is finally absorbed, the kidneys and pericardium 



