FROM TWELVE TO THIRTY-SIX SOMITES 



169 



D.end'l. 



apex, so that by the stage of 29 somites the opening is reduced 

 to a minute ellipse situated on the external side of the dorsalmost 

 portion of the otocyst (see Fig. 93). This portion of the otocyst 

 now begins to form a small conical 

 elevation, and the final closure takes 

 place on the external side of this 

 elevation, which is destined to 

 form the endolymphatic duct. The 

 latter remains united to the epi- 

 dermis at this point for a consid- 

 erable period of time by a strand 

 of cells which may preserve a 

 lumen up to 104 hours (Fig. 98). 

 The final point of closure of the oto- 

 cyst is thus very definitely placed, 

 and it coincides with the middle of 

 the endolymphatic duct, that is, 

 with the junction of the later formed 

 saccus and ductus endolymphaticus. 

 In the Selachia this duct remains 

 in open communication with the 

 exterior throughout life; the rela- 

 tively long persistence of its con- 

 nection with the epidermis in the 

 chick may thus be interpreted as a FIG. 98. Section of the otocyst 

 phylogenic reminiscence of the an- 

 cestral condition. 



The Nose (Olfactory Pits). At 

 about the 28 s stage, the ectoderm 

 on the sides of the head a short dis- 

 tance in front of the eyes appears 

 thickened. Two circular patches of 

 ectoderm are thus marked off, the 



' 



of an embryo of 104 hours. The 

 original opening of the otocyst 

 is drawn out into a narrow ca- 

 nal which connects with the 

 endolymphatic duct (recessus 

 labyrinthi). 



a., Ball of cells in the otocyst 

 (otolith?). b., Canal leading from 

 the surface to the otocyst. D. 

 end'l., Endolymphatic duct. D., 

 Dorsal. Ect., Ectoderm of the 



beginning of the olfactory epithe- surface of the head. Gn., Audi- 

 Hum; at first this grades almost im- Jy7.M^n. L., Lateral. ' M., 



;.,.', . , Median. V., ventral, 



perceptibly into the neighboring 



ectoderm. In the stages immediately following the olfactory 

 plates appear to sink down towards the ventral surface of the 

 head, due no doubt to more rapid growth of the dorsal portion 

 of the head. Thus they appear at the ventro-lateral angles of 



