568 



THE EMHRYOLOGY OF THE FROG 



which they fuse. It is this fusion which causes the depressions in the 

 ectoderm just back of the head on the external surface. 



THE SOMITES 



Just as with the chick, somites are formed by transverse division 

 along the dorsal portion of the embryo, except in the primitive streak 



an: 





Jt- 



A, 



y*- 



ol 



lr.- 



Fig. 331. 



A. Tadpole of the frog at the time of hatching, an, anus ; ex. a, external 

 gills na, nasal pit ; s, sucker ; som, somites ; st, stomodseum ; yk, yolk-sac. 



B. Diagram of a frontal section of a frog larva at the time of hatching, 

 (modified.) c, Crelom ; d, pronephric duct; F, fore-brain; i, infundibulum ; 

 in, intestine; n, nephrostome; o, base of optic stalk; ol, olfactory pit (pla- 

 code) ; p, pharynx ; t, pronephric tubules ; II, hyoid arch ; III -VI, first to fourth 

 branchial arches ; 1, hyomandibular pouch ; 2-6, first to fifth branchial 

 pouches. 



C. A diagram of a transverse section of the frog embryo at the hatching 

 stage, ccel., Crelom ; ect., ectoderm ; int., intestine ; lr., liver ; m.pl., muscle 

 plate ; nch., notochord ; nst., nephrostome ; s.d., segmental duct ; sp.c., spinal 

 cord. The glomeruli are seen opposite the nephrostomes. (A and C, after 

 Borradaile ; B, after Marshall.) 



