202 



STUDY OF YOUNG CHICK EMBRYOS. 



Ao.D. 



cl.II. 



Ao.2. 



card. 



nch. 



cl.II. 



Ph.* 



Ht. 



EC. 



it and the hind-brain appears the cavity of the pharynx, Ph, which on the left 

 side of the embryo shows a prolongation, cl.I, which extends almost to the sur- 

 face of the embryo. This prolongation is the first gill-pouch. On the dorsal side 

 of the pharynx appear the two large aortic trunks, Ao.D, and on its ventral side 

 the two smaller first aortic arches, Ao.i. These are situated in the mandibular 



branchial arch, Mdb, which is well marked 

 externally by a rounded protuberance. The 

 distal end of the second gill-pouch is shown 

 on the right side of the section, d, II. 



Section through the Optic Stalks (Fig. 154). 

 The head of the embryo now appears quite 

 isolated from the body. It is bounded by a 

 distinct layer of ectoderm, EC, and contains 

 the very large fore-brain, f.b, which gives off 

 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 

 anlage of the optic nerve. Between the two 

 optic stalks on the side toward the pharynx 



the floor of the fore-brain bends downward and 



FIG. 154. SECTION OF CHICK EMBRYO WITH 



ABOUT TWENTY-EIGHT SEGMENTS. TRANS- almost joins the superficial ectoderm. All of 



VERSE SERIES 92, SECTION 104. the space between the walls of the fore-brain 



MTrunkoftheaorta. AO.D, Descending aorta. and the t ; c evagination on the. one , hand, 



A 0.2, Second aortic arch, card, Anterior . . 



cardinal vein. cl.II, Second entodermal and of the Superficial ectoderm of the head 



Fore- on the other, is filled with undifferentiated 

 mesenchyma. In this tissue blood-vessels, 

 nerves, lymphatics, and muscles will grow, and. 

 the tissue itself is to produce, the cutis, the 

 subcutaneous tissue, the skull, the dura mater, 



arachnoid membrane, and pia mater. We have in the present undifferentiated 

 stage of this mesenchyma a most striking contrast with the complicated histo- 

 logical 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, Ao, 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 prolongation of its cavity 

 which constitutes the second gill-pouch, cl.II. On the dorsal side of the pharynx 

 '^ending aortas, Ao.D, that on the right of the figure being joined 



f.b. 



gill-pouch. EC, Ectoderm, f.b, 

 brain, h.b, Hind- brain. Ht, Heart, mes, 

 Mesoderm. My, Muscle plate, nch, Noto- 

 chord. Op, Optic vesicle. Ph, Pharynx. 

 X 50 diams. 



