22 OUTLINES OF CHORD ATE DEVELOPMENT 



and sinks slightly below the level of the general ectoderm (Fig. 

 8, A). This strip of cells is the neural plate or medullary plate. 

 It extends from near the anterior (dorsal) margin of the blasto- 

 pore forward, almost to the extremity of the embryo, and back- 

 ward a short distance each side of and partially surrounding 

 the blastoporal opening. The ectoderm bordering the lateral 

 margins of the depressed area becomes slightly elevated, forming 

 the neural ridges or neural folds, and these gradually flow over 

 the margins of the neural plate as this becomes still farther 

 depressed (Fig. 8, J5). Posteriorly the neural folds extend along 

 the sides of the blastopore, and even posteriorly and ventrally 

 to it, completely surrounding it. The neural folds then rapidly 

 approach medially and soon they cover over the neural plate, 

 though separated from it by a shallow space (Fig. 8, C). These 

 processes of depression and roofing over do not occur at once 

 throughout the whole dorsal surface. The depression of the 

 neural plate commences just in front of the blastopore and pro- 

 ceeds thence anteriorly, while the neural folds appear first 

 somewhat further in front of the blastopore and usually, 

 although not invariably, fuse over the neural plate first in the 

 same region. 



Posteriorly the neural folds arising from the lateral and 

 ventral margins of the blastopore, in fusing roof over this 

 structure without closing it, so that the blastopore no longer 

 opens directly to the outside but into the narrow space between 

 the neural plate and the superficial ectoderm layer formed by 

 the fused neural folds (Fig. 6, H). In front of the point where 

 the folds first meet, this space remains widely open upon the 

 surface of the embryo. This opening to the outside is called 

 the neuropore, and as the fusion of the neural folds extends 

 rapidly forward the neuropore is carried along toward the 

 anterior end (Fig. 9, A). By the time of hatching it may be 

 found at almost any point between the middle of the embryo 

 and the anterior margin of the first somite (see below). In 

 this latter region the neuropore remains as a definite opening 

 until the middle of the larval period. There is some variation 

 in different embryos as regards the region of the first fusion of 



