EMBRYOLOGY OF TADPOLE AND CHICK 



565 



manner to that described in the chick, begin fusion in the region of the 

 medulla (Fig. 327), and continue both anteriorly and posteriorly. The 

 transverse fold extends backward so as to form the roof of the expanded 

 brain end of the developing nervous system. It meets the lateral folds 

 in the region lying between the fore and mid-brain, and as it is the 

 last region of the neural tube to close/ it is called the neuropore. The 

 neuropore lies just posterior to where the epiphysis is to appear. It is 

 quite transitory. 



Neural crests are formed in a similar manner to the way they were 

 formed in the chick embryo. 



The blastopore has become a narrow slit, the lateral walls of which 

 fuse, leaving two openings, an upper and a lower. The upper one is 

 directly continuous with the archenteron, while the lower one is known 

 as the proctodaeum. The proctodaeum is but a slight depression lined 

 with ectoderm. 



The posterior ends of the neural folds extend out from the middle 

 regions where they fuse, to form the neural tube which then covers the 

 iipper blastoporal opening, thus forming a connection between arch- 



hi 



Fig. 328. 



Sections of an embryo frog. A, transverse; B, longitudinal, an., 

 Anus., ccel., crclom ; cct., ectoderm or epiblast ; end., endoderm or hypo- 

 blast ; f.br., fore-brain ; h.br., hind-brain ; ht., rudiment of heart ; int., 

 intestine ; l.p., lateral plate of mesoblast ; lr., rudiment of liver ; 7n.br., 

 mid-brain; m.s., mesoblastic somite; mes., mesoblast; mes' ., mesoblast 

 continuous with epiblast of neural canal and hypoblast of notochord ; 

 ne.c., neurenteric canal ; neh., notochord ; ph., pharynx ; pit., rudiment 

 of pituitary body ; so-.m., somatic mesoblast ; sp.c., spinal cord ; sp m., 

 splanchnic mesoblast; stm., stomodaeum. (After Borradaile.) 



