VERTEBRATE DEVELOPMENT 229 



and also a region at the extreme posterior end (proctodaeum) of 

 the alimentary canal where the same condition prevails. In addi- 

 tion to lining the alimentary canal, the endoderm cells also form 

 a number of organs which develop originally as outgrowths from 

 the alimentary canal. These associated organs all form in much 

 the same way by an outpocketing of the endodermal wall of the 

 enteron to form either single or paired structures which later dif- 

 ferentiate into various important organs, such as the lungs, 



THYROID GLANDS, LIVER, PANCREAS, and BLADDER. (W. fs. 110 ; 



174, K.) 



From the important mesoderm layer are formed the muscular, 

 vascular, and connective tissues of the body. Previous men- 

 tion has been made of the division of the mesoderm into the verte- 

 bral and lateral plates. The myotomes of the former, together 

 with the somatic layer of the lateral plates, give rise to the muscle 

 tissue of the body wall and appendages, together with the connec- 

 tive tissues, vascular elements, etc. ; while the muscle layers, and 

 other elements of the wall of the alimentary canal, except the endo- 

 dermal lining, are derived from the splanchnic layer. The coelom 

 is lined with peritoneum which is continuous over the alimentary 

 canal and other organs. Both the somatic and splanchnic layers 

 contribute to the formation of the peritoneum. From it arise the 

 mesenteries by which various organs are attached to, and sus- 

 pended from, the walls of the coelom. The mesoderm also gives 

 rise, for the most part, to the urogenital system. (W. f. 95, B.) 



The fact should be emphasized that the organs, in general, are 

 not formed from a single tissue, but on the contrary represent a 

 mosaic of various tissues. The liver, for example, is fundamentally 

 an endodermal organ, but not exclusively so, for it contains vas- 

 cular, connective, and nervous elements derived from both the 

 mesoderm and ectoderm. 



5. Metamorphosis 



After a period of time, the length of which shows a great deal of 

 variation among different species of Frogs, the fish-like larva, or 

 tadpole, undergoes radical changes both in structure and habits. 

 These changes constitute a process of metamorphosis, and result 

 in the formation of an air-breathing, adult Frog with two pairs of 

 limbs and no tail. Experimental work in recent years has shown 

 that the process of metamorphosis in the Frog is controlled to a 



