THE DEVELOPMENT OF THE FROG 263 



is formed from three germinal layers, which are already in 

 part differentiated. The central nervous system has clearly 

 been formed from the outer layer or epiblast, the rest of the 

 layer forming the external covering of the body. The meso- 

 blast and hypoblast were formed in close connection with 

 the blastopore, and at first were blended together, but soon 

 separated out, and the notochord was formed in the mid- 

 dorsal line, where the two layers remained longest in contact. 

 There is still a large mass of yolk occupying the posterior 

 end of the enteron ; it must be regarded as belonging to the 

 inner layer or hypoblast. All these relations, and the growth 



Fig. 63 



Diagrammatic longitudinal section through .-in embryo of about the same age 

 as that drawn in fig. 60, C. an, anus ; tut, enteron ; fbr, fore-brain ; h.br, 

 hind-brain; ht, heart; hy, hypoblast; /;, liver diverticulum; m.br, mid-brain; 

 ines, ventral mesoblast ; nc, remains of the neurenteric canal ; nch, notochord ; 

 fit, epibhistic ingrowth which will form the pituitary body ; sp.c, points to the 

 canal of the spinal cord; yk, yolk. The external epiblast is represented by a 

 thick black line, the nervous system is shaded with dots. 



processes that led up to them, should be compared with the 

 stages in the development of Amphioxus. 



The further development of the embryo is too complex 

 to follow as a whole, and it is necessary to deal with it organ 

 by organ. It would be beyond the limits of this work to 

 attempt a detailed account of the organogeny of the frog, 

 and in the following sketch of the course of events, only 



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