410 DEVELOPMENT OF FROG AND OTHER VERTEBRATA 



blastopore originally occupies a place near the equator of the 

 spherical mass as it is oriented with reference to gravity. The 

 extension of the crescentic blastopore and the encircling of the 

 yolk-laden cells carries this dorsal lip downward through an angle 

 of about ninety degrees {cf. Fig. 213 G to I), until its position 

 about coincides with the vegetative pole. By this time, how- 

 ever, the development of the enteron and the reduction of the 

 yolk-laden cells produce a change in the center of gravity, such 

 that the embryo gradually rotates within its gelatinous capsule, 

 bringing the blastopore and vegetative pole to a position at the 

 posterior end of the embryo (Fig. 213 I). By growth of the dorsal 

 region, the area representing the animal pole is gradually shifted 

 to the antero-ventral surface of the tadpole. 



The Neural-fold and Early-embryo Stages. — During the later 

 stages of gastrulation other changes are in progress that have been 

 omitted from the foregoing account for the sake of simplification. 

 The outermost layer of ectoderm cells becomes tliickened in a 

 broad region, the neural plate (cf. Figs. 215 and 216), extending 

 anteriorly from the blastopore along the future doi-sal side of the 

 embiyo and terminating in a broadly rounded outline. As the 

 gastrula develops into the neural-fold stage, which succeeds it, this 

 thickening becomes more pronounced at the edges and forms lateral 

 elevations which later rise above the surface as the neural folds or 

 ridges. These become continuous at the anterior end of the 

 neural plate and extend posteriorly to either side of the blastopore. 

 Since the neural plate eventually becomes the central nervous 

 system, the dorsal, ventral, anterior, and posterior regions of the 

 future adult become increasingly evident from this point onward. 

 Slightly elevated areas, known as the sensory plate and the gill 

 plate, may also be recognized in such an embryo. 



In subsequent stages the neural folds increase in height, and 

 a furrow, the neural groove {cf. Fig-s. 215 and 216) appeare in the 

 median region of the neural plate. As this neural groove becomes 

 deeper, the neural folds rise up laterally as ridges that increase 

 rapidly in height and grow toward one another on the mid- 

 hne until they meet and fuse to form the neural tube, which is 

 the rudiment of the brain and spinal cord. The manner in which 

 this infolding is accomplished is clearly seen in transvei-se sec- 

 tions. As this fusion of the neural fokls occure fii"st in the 

 middle region, the neural tube remains open for a time at its broad 



