io6 



THE BIOLOGY OF THE FROG 



CHAP. 



The lining of the mouth cavity is formed from an invagin- 

 ation of ectoderm, the stomodeum, which pushes in until it 

 breaks through into the archenteron. A similar ectodermal 

 invagination, the proctodeum, forms the lining of a small part 

 of the posterior end of the alimentary canal. The lens and 



Fig. 26. — Cross section of a frog embryo. A R, archenteron ; MS, meso- 

 blastic somites ; A T , notochord ; NS, neural crest ; M, medullary tube; 

 PR, pronephros ; SN t subnotochordal rod ; SO, SP, somatic and splanch- 

 nic mesoderm. (From Morgan, after Marshall.) 



cornea as well as the retina of the eye, and the vesicle of 

 the inner ear, also take their origin from this layer. 



Organs from the Entoderm. — The entoderm, or the 

 germ layer which is invaginated within the egg, gives rise to 

 the lining of the alimentary canal and of all organs which 



