516 BERTRAM G. SMITH 



now suggest the definitive primary divisions (forebrain, midbrain 

 and hindbrain) of the embryonic brain. 



The various structures of the neural plate have loot yet been 

 followed into the definitive divisions of the embryonic and adult 

 brain; but the preliminary examination of some later embryos 

 dissected by splitting them in the median line with a razor shows 

 that the transverse divisions in the neural plate persist for some 

 time after the closure of the neural folds. Neuromeres in the 

 closed neural tube are also often apparent from the surface. Hence 

 it is easy to judge approximately concerning the fate of individ- 

 ual neuromeres of the cephalic plate, but to avoid possible error 

 it seems best to defer a definite statement until the internal his- 

 tory of the brain has been more carefully studied. 



The pair of folds which in the preceding stages extended trans- 

 versely on each side of the cephalic plate now slant backward 

 (see especially fig. 185); the appearance, particularly in living 

 material, suggests that they are in some way concerned with 

 the origin of the vascular bands which in later stages extend 

 along each side of the yolk sac and give rise to the omphalo- 

 mesenteric or vitelline veins (fig. 192). 



The transverse opacity in front of the neural plate is conspic- 

 uous in living material viewed by transmitted light (fig. 186), 

 but is not apparent in surface views of preserved material. 



The anterior part of the blastopore is now normally closed 

 over, and the posterior or ventral part is reduced to a transverse 

 slit (figs. 188 and 189). Apparently the middle portion of this 

 transverse slit never becomes completely closed, but in later 

 stages persists as the anal or cloacal opening. The embryonic 

 body has elongated so that it now extends over about half the 

 circumference of the egg (fig. 187). 



For the study of transverse divisions in the open neural plate, 

 Necturus is not nearly so favorable as Cryptobranchus. In 

 Necturus the blastopore (figs. 268 to 279) closes much earlier 

 than in Cryptobranchus. Moreover in Necturus the closure 

 of the blastopore is often practically complete ; in many specimens 

 preserved at the time of the closure of the neural folds, scarcely 

 more than a vestige of the blastopore is visible from the surface. 



