430 DEVELOPMENT OF FROG AND OTHER VERTEBRATA 



In brief, it may be said that the development of birds is funda- 

 mentally like that of the frog, as shown by such details as the 

 origin and development of the mesoderm; the neurenteric canal, 

 which occurs in reptiles and in some of the birds; the notochord, 

 which is later surrounded by the vertebral column; the stomodseum 

 and proctodseum; and by such larger features as the origin of the 

 various organs from the germ layers and the manner in which the 

 circulatory system is formed. This system is at first fish-like, with 

 its two-chambered heart (Fig. 226) and arteries passing in the gill 

 bars between gill slits, but is later modified by the appearance of 

 the four-chambered heart of the adult bird and other parts adapted 



Fig. 226. — Three stages in development of heart in chick, showing transition 

 from two-chambered to four-chambered condition. 



a, auricle; I. a., left auricle; l.v., left ventricle; r.a., right auricle; r.v., right ventricle; 

 t, truncus arteriosus; v, ventricle. (Drawn by Wiley Crawford.) 



for respiration by means of lungs. Features of the development of 

 birds and reptiles that are entirely different from anything found 

 in the Amphibia are the embryonic membranes. It will be neces- 

 sary to explain these structures in a general way in order that we 

 may understand the similar features of mammalian development. 

 Although these embryonic membranes of birds and reptiles 

 present complexities of structure that cannot be described here, it 

 will be possible to explain their schematic arrangement as they 

 appear in one plane of section. In Fig. 227 A is shown a longitu- 

 dinal section of a chick embiyo in its relation to the yolk and shell, 

 at a stage when the neural tube is beginning to be differentiated 

 into the spinal cord and brain, while the head and tail regions of 

 the animal are becoming apparent. The embryo is, of course. 



