EARLY DEVELOPMENT OF THE FROG. 



63 



the ventral wall of the archenteron (gut) (Fig. 36) . This growth in length is the 

 beginning of the characteristic cephalo-caudal elongation of the larva and 

 adult. It is due in part to proliferation of cells generally but chiefly due to 

 proliferation at the caudal end in the region dorsal to the blastopore. . Here 

 as in Amphioxus growth takes place largely from before backward; and the 

 bud-like process in the dorso-caudal region is one of the outward expressions 

 of the growth. 



nf nf 



9 





FIG. 36. Postero-lateral views of successive stages following gastrulation in the frog. Ziegler, 

 from Kellicott. A, blastopore in process of closing, neural folds slightly indicated; B, 

 gastrula slightly elongated, blastopore closed, neural groove and folds obvious; C, anal 

 portion of blastopore still visible at bottom of proctodaeum, neural folds closing dorsally; 

 D, neural folds nearly closed, branchial arches appearing, tail bud forming; E, neural folds 

 fused, tail bud more conspicuous. 



b, Blastopore containing yolk plug; bi, dorsal part of blastopore (rudiment of neurenteric canal) ; 

 fe, ventral part of 'blastopore (rudiment of anus); ba, branchial arches; g, neural gro ve ; nf, 

 neural folds; np, neural plate; p, proctodaeum, with anal portion of the blastopore at the 

 bottom; s, oral sucker; t, tail bud; x, neural folds covering the blastopore thus establishing 

 the neurenteric canal. 



In order to bring the development of the frog up to a point corresponding 

 to the stage of Amphioxus reached at the end of the previous chapter, it is 



