200 



STUDY OF YOUNG CHICK EMBRYOS. 



which is elsewhere quite thin. The ectoderm, EC, covering the dorsal side of the 

 hind-brain is very thin, but the portion in front of the auditory invagination is 

 somewhat thicker. The ectoderm of the invagination is very much thickened and 

 contains numerous somewhat crowded nuclei at all levels. These nuclei are rounded 

 in form and have one or two very distinct nucleoli. On the posterior side of the 

 otocyst there is very little mesoderm; on the anterior side, much more. Between the 



Epen. 

 /f " ^K ^-"' 



h.b. __ M Ec 



Ot.d 



nch. 



Ao. 



EC 



FIG. 151. SECTION OF CHICK EMBRYO WITH ABOUT 

 TWENTY-EIGHT SEGMENTS. TRANSVERSE 

 SERIES 92, SECTION 73. 



Ao, Aorta. EC, Ectoderm. f.b, Fore-brain, h.b, 

 Hind-brain. mes, Mesoderm. nch, Noto- 

 chord. Ot, Otocyst. Vc, Vein. X 50 diams. 



FIG. 152. SECTION OF CHICK EMBRYO WITH ABOUT 

 TWENTY-EIGHT SEGMENTS. TRANSVERSE 

 SERIES 92, SECTION 83. 



A o.i, Prolongation toward the fore-brain of the first 

 aortic arch. Ao.D, Descending aorta, card, 

 Anterior cardinal vein, cl.pl, Closing plate. 

 cl.i, First gill-pouch. EC, Ectoderm. Epet. . 

 Roof of hind- brain. /.&, Fore-brain, h.b, 

 Hind-brain, "mes, Mesoderm. nch, Notochord 

 Op, Optic vesicle. Ot.d, Right otocyst. Ot.s, 

 Left otocyst. Ph, Pharynx. X 50 diams. 



developing otocyst and the notochord there is a blood-vessel, Ve, with merely endo- 

 thelial walls, a branch of the cardinal vein. Between the hind-brain and fore- 

 brain near the notochord, the two aortae, Ao, are cut. In their interior there can 

 usually be seen a certain number of nucleated blood-cells varying somewhat in 

 size and appearance, but generally having a rounded form with distinct outline and 

 a well-defined nucleolated nucleus. 



Section through the Left Auditory Invagination (Fig. 152). Owing to the irregu- 

 lar form of the embryo the sections through the otocyst are not symmetrical. The 



