530 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



\vith the hypoblast, and forms the greater part of the 

 wall of the permanent digestive tract. In the Yerte- 

 brata the upper dorsal portion of the mesoblast 

 surrounds the notochord, and gives rise to the 

 rudiments of the bodies of the vertebrae ; the rest 

 of the dorsal portion is converted into the so-called 

 muscle plates. Other parts of the mesoblast give rise, 

 in various ways, to the heart and blood-vessels, when 

 such are present, to the blood itself, and to the renal 

 and generative organs. 



The hypoblast, in addition to forming the lining of 

 a more or less large portion of the digestive tract, and 

 of the organs (liver, lungs) which are developed as 

 outgrowths from it, does, in the Cephalochordata and 

 Urochordata at any rate, give rise also to the noto- 

 chord, which arises as a diverticulum or outgrowth 

 on the dorsal side of the primitive enteron. 



We have, then, 



(1) The epiblast giving rise to the epidermis, 

 nervous system, stomodseum, and proctodaeum. 



(2) The mesoblast gives rise to the internal skeleton, 

 the muscles, connective tissue, vascular, renal, and 

 generative systems. 



(3) The hypoblast forming the lining of the diges- 

 tive tract and its appendages, and, in the lower Chor- 

 data, the notochord. 



While in a large number of Metazoa the fertilised 

 ovum, by a regular and steady series of differentia- 

 tions, gives rise to forms which essentially resemble 

 their parents, there is a not inconsiderable number of 

 cases in which the young live for a time an inde- 

 pendent life under a form very different to that of the 

 adult. Such forms are known as larvae ; a few only 

 can be here dealt with. 



Among the Cliorclata the best known is the 

 tadpole stage of the frog ; instead of a tailless four- 

 limbed animal with a large mouth, no gills or gill 



