284 



THE CHICK. 



MN), which forms the basis of the lower jaw. The second and 

 widest arch is the hyoid arch, HY ; and behind this come the first, 

 second, and third branchial arches, the hindmost or third branchial 

 arch being immediately behind the last or third branchial cleft. 

 These visceral arches, and the gill-pouches separating them 

 from one another, correspond exactly with the similarly named 



MP 



FIG. 124. A section through the head of a Chick Embryo at the end of the 

 third day of incubation, the section being taken along a plane indicated 

 by the two arrows and crosses in Fig. 11 3, p. 253. The right side of the 

 section is at a level slightly dorsal to that of the left side, x 30. 



A, dorsal aorta. Al, first aortic arch, in the mandibular arch. A2, second aortic 

 arch, in the hyoid arch. A3, third aortic arch, in the first branchial arch. AC, carotid 

 artery. AI, internal carotid artery. BM, cavity of mid-brain. BR1, first branchial 

 arch. CH, notochord. HM, hyomandibular cleft. HY, hyoid arch. MK", mandi- 

 bular arch. MP, muscle plate. NE, ganglion of spinal nerve. NS, spinal cord. 

 TP, pharynx. VB, anterior cardinal vein. 



structures in the tadpole ; the sole difference of importance being 

 that in the chick no gills are, at any period, developed in con- 

 nection with them. The fact that these structures, which are 

 only intelligible through their association with aquatic respira- 

 tion, are present in the early developmental stages of the chick, 

 must be held to prove the descent of birds from aquatic, gill- 

 breathing ancestors. 



