EMBRYO WITH TWENTY-EIGHT SEGMENTS. 



211 



Ao.D. 



el. II. 



A 0.2. 



card. 



nek. 



cl. II 



Section through the Optic Stalks (Fig. 156). — The head of the embryo now ap- 

 pears quite isolated from the body. It is bounded by a distinct layer of ectoderm, 

 Ec, and containsthe very large fore-brain, f.b, which givesoff on either side an optic 

 evagination, Op, the walls of which are quite thick, about the same as those of the 

 fore-brain proper. Each optic evagination is widest toward the side of the head 

 and is constricted toward the brain, with 

 which, therefore, it is connected by a stalk 

 in which we can already recognize the an- 

 lage of the optic nerve. Between the two 

 optic stalks on the side toward the pharynx 

 the floor of the fore-brain bends downward 

 and almost joins the superficial ectoderm. 

 All of the space between the walls of the 

 fore-brain and the optic evagination on 

 the one hand, and of the superficial ecto- 

 derm of the head on the other, is filled 

 with undifferentiated mesenchyma. In 

 this tissue blood-vessels, nerves, lymph- 

 atics, and muscles will grow, and the tis- 

 sue itself is to produce the cutis, the sub- 

 cutaneous tissue, the skull, the dura mater, 

 arachnoid membrane, and pia mater. We 

 have in the present undifferentiated stage 

 of this mesenchyma a most striking con- 

 trast with the complicated histological 

 conditions of the adult. The opposite 

 part of the embryo represents the cervical 

 region. At one side we see a small piece 

 of the heart appearing, Ht, and higher up 

 is the wide pharynx, Ph, underneath which 

 is a blood-vessel, A6, the main aorta. . To 

 the left appears another blood-vessel, Ao. 

 2, a portion of the second aortic arch. 

 The pharynx shows on one side the pro- 

 longation of its cavity which constitutes the second gill pouch, cl. II. On the 

 dorsal side of the pharynx appear the descending aorta?, Ao. D, that on the right 

 of the figure being joined by the third aortic arch, near which appears an accumu- 

 lation of more deeply colored cells, cl. II, part of the entodermal wall of the second 

 gill pouch. Between the pharynx and the hind-brain we have a round section of 



Fig. 156. — Section of Chick Embryo with 

 about Twenty-eight Segments. Trans- 

 verse Series 92, Section 104. 



Ao, Trunk of the aorta. Ao.D, Descending aorta. 

 A0.2, Second aortic arch, card, Anterior 

 cardinal vein. cl.II, Second entodermal gill 

 pouch. Ec, Ectoderm, f.b, Fore-brain. 

 h.b, Hind-brain. Hi, Heart, mes, Meso- 

 derm. My, Muscle plate, nch, Notochord. 

 Op, Optic vesicle. Ph, Pharynx. X 5° 

 diams. 



