THE BRAIN. 



253 



The medulla oblongata is formed from the hind-brain, the 

 central canal of this part of the brain becoming the fourth 

 ventricle of the adult. 



BL 



Fig. 113. — A Chick Embryo at the end of the third day of incubation. Owing 

 to the twisting of the fore part of the embryo, the head and neck are seen 

 from the right side, and the hinder part of the body from the dorsal 

 surface. The amnion has been removed. (^Cf. Fig. 99.) x 20. 



A, dorsal aorta. Al. first or mandibular aortic arch. A3, tliird aortic arch, in the 

 first branchial arch". AC, carotid artery. AV, vitelline artery. BF,thalameneeplialou 

 or fore-brain. BH medulla oblongata. BL, cerebelliun. BM, mid-brain. BS, 

 cerebral hemisphere. EI, auditory vesicle. HCl, fii'st branchial cleft. HC2, second 

 branchial cleft. HM, hyo-mandibular cleft. MS, mesoblastic somite or protovertebra. 

 WS, spinal cord. OC. optic cup. OF, olfactory pit. OL, lens. PW, pineal body. 

 H, A, auricle of heart. RT, truncus arteriosus. RV, ventricle of heart. VV", viteUiue 

 veins. 



The two arrows and crosses indicate the plane along which the section shown in 

 Fig. 124 is taken. 



The walls of the medulla oblongata are at first of nearly- 

 equal thickness all round ; but before the end of the second day 



