DEVELOPMENT OF RANA 179 



mesoderm and notochord from the wall of the archenteron 

 begins in the anterior region of the embryo ; farther back they 

 are not distinct, and merge into the rim of the blastopore. 

 The rim of the blastopore may indeed be defined as the region 

 where the ectoderm, mesoderm, endoderm and notochord are 

 all in contact with a zone of actively growing cells, which 

 contributes new tissue to each layer. 



The new mesoderm formed from the blastopore-rim is 



Fig. 79. — Transverse section through an embryo of Rana slightly older than 

 the previous, showing the complete separation of the notochord («) 

 from the endodermal wall (en) of the gut (g). 



The coelom (c) has arisen as a split in the mesoderm (m), which forms a 

 somite (ms) on each side of the notochord. The neural folds («/) have closed 

 over the groove converting it into the nerve-tube (nc)> on each side of which 

 are the neural crests (ncr). ec, ectoderm ; y, yolk cells. 



peristomial, that split off from the wall of the archenteron 

 farther forward, is called gastral mesoderm. Apart from their 

 method of formation, there is no difference between these two 

 kinds of mesodermal tissue. 



This activity of the lip or rim of the blastopore is a continua- 

 tion of the process of epiboly, and its result is to produce an 

 elongation of the embryo. Eventually the blastopore becomes 

 oval and slit-like by the apposition to one another of its lateral 



