360 



INVEETEBEATA 



CHAP. 



As development proceeds the upper portion of the egg, covered by 

 the blastoderm, begins to be separated by a constriction from the 

 lower part, which consists merely of yolk covered by the yolk- 

 membrane, and so we are enabled 

 to distinguish between "embryo" 

 ;ind yolk-sac. It must be remem- 

 l)ered, however, that a considerable 

 portion of the yolk is contained 

 within the confines of the embryo ; 

 this is known as the internal yolk- 

 sac, and is disconnected from the 

 external yolk-sac, which appears 

 as an appendage. The two are, of 

 course, joined by a neck of com- 

 inunicatioii. 



Fio. 591. — Embryo of Loligo vulgaris seeu 

 from the side and behind in order to 

 illustrate the development of the ganglia. 

 The embryo is younger than that repre- 

 sented in Fig. 289. (After Faussek.) 



A, Embryo seen from behind. B, Embryo 

 seen from the side, a.//, anterior fimnel folds ; 

 ar, rudiments of arms ; hr, rudiment of gill ; eg, 

 rudiment of cerebral ganglion ; oo, eye-cup ; o.st, 

 eye-stalk ; o(, otocyst ; jj././, posterior funnel 

 folds ; p.g, rudiment of pedal ganglion ; s.s, shell- 

 sac ; v.g, visceral ganglion. 



Fig. 292. — Embryo of Loligo mdgaris in- 

 the stage termed by Fanssek Stage 2, 

 wlien the embryo begins to be grooved 

 otf from the yollc-sac ; posterior view. 

 (After Korschelt.) 



Letters 'as in previous iigure. In addition, 

 u.p, anal papilla ; r./, retractor muscle of 

 funnel ; t.ar, tentacular arms. 



When the grooving off of the external yolk-sac has become 

 distinct, many other changes take place. The eye-stalks grow in 

 length, while the eye-pits become closed, and the inner segment of 

 the lens is developed as a secretion from the anterior wall of each 

 pit. The otocysts also become closed and sink into the ridges which 



