228 



PRINCIPLES OF ANIMAL BIOLO(JY 



tlividcs into two parts (B, C, D). As these grow in size they become 

 branched. The undivided stalk of the lung rudiment is the trachea, the 

 two principal branches are the bronchi, and the finer divisions are the air 

 passages within the lungs. Mesoderm is constantly pushed before the 

 growing lung rudiments, so that 

 the adult lungs are invested 

 with a peritoneum. Other 

 mesodermal tissue is incorpo- 

 rated in the lungs, among the air 

 passages, where blood vessels are 

 abundantly developed. 



Fig. 183. Fig. 184. 



Fig. 183. — Diagram illustrating the development of some of the organs of a vertebrate 

 animal as viewed from the side. The section is taken a trifle to one side of the median 

 plane. The stages shown are not all contemporaneous, br, brain; c, coelom; int, intestine; 

 li, liver; 7n, mouth; ms, mesoderm; nd, notochord; pc, pericardial chamber; sp, spinal 

 cord; st, stomach. 



Fig. 184. — Diagram representing the development of some of the organs of vertebrate 

 animals, at a later stage than in Fig. 183. The figure is a trifle to one side of the median 

 plane. The stages shown are not necessarily contemporaneous, an, anus; hd, bile duct; 

 br, brain; c, ccelom; dp, dorsal rudiment of pancreas; gb, gall bladder; int, intestine; lo, 

 lung; H, liver; m, mouth; ms, mesoderm; nd, notochord; pc, pericardial chamber; rt, root of 

 tongue; sp, spinal cord; st, stomach; t, tongue; vp, ventral rudiment of pancreas. 



Minor Features of Digestive Tract. — The tongue begins in a group of 

 elevations from the floor and sides of the mouth. Two of these eleva- 

 tions are represented in Fig. 184. 



The anus develops, in a few animals, directly from the blastopore, 

 and in them is never closed. In other animals, in which the blastopore 



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