360 



INVEKTEBRATA 



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 " 

 and yolk-sac. It must be remem- 

 bered, 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 

 C 9 external yolk-sac, which appears 

 as an appendage. The two are, of 

 course, joined by a neck of corn- 

 er HL nmuication. 



ost 



r 



oc 



B 



FIG. 291. Embryo of Lulif/o vulgaris seen 

 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 Fanssek.) 



A, Embryo seen from behind. 13, Embryo 

 seen from the side, n.f.f, anterior funnel folds ; 

 ar, rudiments of arms ; In; rudiment of gill ; c.g, 

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

 eye-stalk ; ot, otocyst ; />././, posterior funnel 

 folds ; /..-/, rudiment of pedal ganglion ; s.s, shell- 

 sac ; <:<i, visceral ganglion. 



Fi<;. 292. Embryo of Lnliyti nili/nri* in 

 the stage termed by Fanssek Stage 2, 

 when the embryo begins to be grooved 

 oil' from the yolk-sac ; posterior view. 

 (After Korschelt.) 



Li'ttiT.s as in previous figure. In addition, 

 it. p, anal papilla ; r.f, retractor muscle of 

 funnel ; t.ai; 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 



