THE EARLY DEVELOPMENT OF THE FROG 117 



characteristics of this period are associated with the develop- 

 ment of this system. The transverse neural fold, which marks 

 the anterior limit of the central nervous system (Figs. 22, C; 32, 

 G), forms from materials located in about the middle of the roof 

 of the blastocoel, while the posterior limit of the nervous system 

 is just above or in front of the dorsal lip of the blast o pore. The 

 downward extension of the latter, on account of the confluence, 

 increases the length of the rudiment of the nervous system, so 

 that before the blastoporal margin or germ ring has fully con- 

 tracted, it extends nearly from pole to pole, around the posterior 

 side of the gastrula. The rotation of the gastrula then changes 

 the apparent, though not the true morphological, position of the 

 anterior margin of the medullary plate, so that when the trans- 

 verse neural fold actually appears , it is on the anterior side of 

 the gastrula, and the medullary plate itself occupies nearly the 

 whole dorsal surface of the early embryo (Fig. 32, G). We have 

 already described the formation of the neural or medullary 

 plate, the neural groove, and the lateral and transverse neural 

 or medullary ridges. 



The elevation of the neural ridges, particularly their anterior 

 >rtions, soon becomes very marked, and as it continues the 

 liddle of the plate sinks downward and soon the ridges bend 

 )ver toward each other and meet along the mid-line, where they 

 fuse, transforming the neural plate into the neural tube contain- 

 ig the neural canal or neuroccel (Figs. 22, D, E; 38) . For a long 

 time after fusion a deep median groove marks the region where 

 ie folds have come together. The fusion of the neural ridges 

 loes not occur simultaneously throughout their extent, but 

 ret in about their middle, then extending posteriorly and 

 tteriorly from this region (Fig. 38). This is approximately 

 ie location of the future medulla (myelencephalon) ; from this 

 ;ime, therefore, the spinal cord and brain are distinguishable 

 37). The narrower cord region closes before the much 

 dder brain. In the closure of the brain region the transverse 

 )ld plays an important part; this extends backward, roofing the 

 expanded cavity of the brain, and meets the slowly fusing 

 iteral folds in the region between the future fore- and mid- 



