STRUCTURE OF THE VERTEBRATES 55 



The stages in the development of the notochord are telescoped. 

 Instead of infolding as in Amphioxus, the endodermal cells mul- 

 tiply rapidly along the dorsal line of the archenteron and split 

 off to form the continuous rod of notochordal tissue. The 7nes- 

 oderm also goes through a compressed development. Unlike the 

 metameric enterocoels of Amphioxus, the frog mesoderm arises 

 as a proliferation of cells on either side of the notochord, the 

 continuous band of tissue soon dividing into metameric blocks. 

 A cavity develops within each mesodermal mass, homologous 

 with the enterocoelic cavities of Amphioxus. Later development 

 of the organs is sufficiently similar to that of Amphioxus not to 

 need further discussion in a text on anatomy. 



As the embryo grows the nervous system and the organs of 

 special sense develop. The mouth breaks through, and the em- 

 bryo, now a young larva, ceases to depend upon its yolk mass 

 for food and begins eating. Up to this time respiration has been 

 through the epidermis to the blood of the already functioning 

 vascular system, but as the larva develops in mass gills develop 

 in correlation with changes in the branchial arches. These gills 

 are external and three in number. In time the external gills are 

 lost, internal ones having developed within the gill slits, and the 

 frog tadpole pursues a fish-like existence in the water. Hind legs 

 make their appearance, getting larger as the tadpole grows older. 



The larval stage varies in length in different species, lasting 

 from six weeks in the wood frog to as much as three years in the 

 bull frog. Temperature causes a variation in time, heat speed- 

 ing metamorphic processes. When the tadpole approaches meta- 

 morphosis it stops eating; the front legs, which have been en- 

 closed in an atrium analogous with that of Amphioxus, break 

 through; the gills are lost and the animal makes more use of its 

 lungs; and the tail is absorbed and used for food. When meta- 

 morphosis is complete the young frog hops out on land, weighing 

 approximately half as much as did the tadpole. From this time 

 until maturity development is only a matter of growth and dif- 

 ferentiation of certain organs, as in the land vertebrates. 



