EMBRYO WITH TWENTY-EIGHT SEGMENTS. 



201 



Epen. 



card. 



sent section shows the opening of the left otocyst, Ot.s, and a closed section 

 of the right otocyst, Ot.d. At its lower inner edge the outer boundary of the wall 

 of the otocyst is indistinct, this arppearance being due to the union of the cells of 

 the acoustic ganglion with the wall of the otocyst. The section also passes through 

 the first gill-cleft, cl.i, of the right side, and shows very distinctly indeed the 

 closing plate, cl.pl, which is formed by a 

 fusion of the ectodermal and entodermal 

 cells. On the opposite side of the section 

 the same cleft is imperfectly shown. On 

 the posterior side of the cleft is the dorsal 

 aorta, Ao.D, and on the anterior side of 

 the cleft, extending toward the fore-brain, 

 f.b, appear the sections of the two pro- 

 longations of the first arches, Ao.i, toward 

 the fore-brain. In this specimen each pro- 

 longation forms a loop, which rejoins its 

 arch* dorsally. In the region of the fore- 

 'brain appears a shaving from the edge of 

 the optic evagination, Op. The anterior 

 cardinal veins, card, appear just inside of 

 the otocyst close to the ventral wall of the 

 hind-brain, h.b. 



Section through the Invagination of the 

 Optic Lens (Fig. 153). This section also 

 passes through the hind-brain, h.b, fore- 

 brain, f.b, and through the openings of FIG. 153. -SECTION OE CHICK EMBRYO WITH ABOUT 

 J TWENTY-EIGHT SEGMENTS. TRANSVERSE 



both invaginations to form the anlages, L, SERIES 92, SECTION 96. 



of the lenses Of the eye. These invagina- Ao.D, Descending aorta. Ao.i, First aortic arch: 



Mdb. 



Ent. 



Ret. 



f.b. 



Ao.2, Second aortic arch, card, Anterior car- 

 dinal vein. cl.I, First gill-cleft. cl.II, Second 

 entodermal gill-cleft. EC, Ectoderm. Ent, 

 Entoderm. Epen, Roof of hind-brain, f.b, 

 Fore-brain, h.b, Hind-brain. L, Invagination 

 of lens. Mdb, Mandibular arch, mes, Meso- 

 defm: .nch, Notochord. Op, Optic vesicle. 

 Ph, Pharynx. Ret, Retina. X 50 diams. 



tions bear a striking resemblance to those 



which form the otocysts. The ectoderm, 



EC, over the roof of the fore-brain is very 



thin and passes abruptly into the thickened 



layer which forms the wall of the invagina- 



tion. On the ventral side the ectoderm is 



somewhat thicker. The wall of the lentic 



vesicle is quite thick; its nuclei are numerous, but are situated chiefly on the meso- 



dermal side of the layer, so that toward its outer surface the layer is comparatively 



free from nuclei. The invagination of the lens rests against the optic vesicle, the 



wall of which, Ret, next to the lens is thicker than the posterior or inner wall of the 



optic vesicle. The thickened outer portion is the anlage of the retina, the thinner inner 



portion is the anlage of the pigment layer covering the retina. The fore-brain, 



f.b, has an elongated form with quite thick walls crowded with nuclo' Between 





