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 blastoccel, while the posterior limit of the nervous system 

 is just above or in front of the dorsal lip of the blastopore. 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 

 portions, soon becomes very marked, and as it continues the 

 middle of the plate sinks downward and soon the ridges bend 

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

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

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

 time after fusion a deep median groove marks the region where 

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

 does not occur simultaneously throughout their extent, but 

 first in about their middle, then extending posteriorly and 

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

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

 time, therefore, the spinal cord and brain are distinguishable 

 (Fig. 37). The narrower cord region closes before the much 

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

 fold plays an important part; this extends backward, roofing the 

 expanded cavity of the brain, and meets the slowly fusing 

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



