276 



STUDY OF YOUNG CHICK EMHRYOS. 



Epen. 



card. 



cl.I. 



Mdb. 



Ent. 



Ret. 



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. h. 



Section through the Invagination of the Optic Lens (Fig. 155). — This section 

 also passes through the hind-brain, h.h, fore-brain, /. 6, and through the openings 



of both invaginations to form the lens, L, 

 which bears a striking resemblance to 

 the invagination which forms the oto- 

 cyst. 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 invagination. On 

 the ventral side the ectoderm is some- 

 what thicker. The wall of the lentic 

 vesicle is quite thick, its nuclei are 

 numerous, but are situated chiefly on 

 the mesodermal 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 pos- 

 terior or inner wall of the optic vesicle. 

 The thickened outer portion is the an- 

 lage of the retina, the thinner inner 

 portion is the anlage of the pigment 

 layer of the retina. The fore-brain, 

 /. b, has an elongated form with quite 

 thick walls crowded with nuclei. Be- 

 tween it and the hind-brain appears the 

 cavity of the pharynx, Ph, which on the 

 left side of the embryo shows a prolon- 

 gation, cl. I, which extends almost to 

 the surface of the embryo. This prolon- 

 gation is the first gill pouch. On th'^ 

 dorsal side of the pharynx appear the two large aortic trunks, Ao. D, and ors 

 ventral side the two smaller first aortic arches, Ao. i. These are situated iihe 

 mandibular branchial arch, Mdb, which is well marked externally by a roued 

 protuberance. The second gill pouch is shown on the left-hand side of thec- 

 tion, cl. II. 



h.b. 



nch. 

 cl. II. 



Ao D. 



A0.2. 



Ao./. 



op. 



f.b. 



Fig. 155. — Section of Chick Embryo with 

 ABOUT Twenty-eight Segments. Trans- 

 verse Series 92, Section 96. 



Ao.D, Descending aorta. Ao.i, First aortic arch. 



, A0.2, Second aortic arch, card, Anterior car- 

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

 entodermal gill cleft. Ec, Ectoderm. Etit, 

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

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

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

 derm. Hc/i, Notochord. Op, Optic vesicle. 

 P/i, Pharynx. Het, Retina. X 5° diams. 



