. A manual of elementary zoology . Zoology. 492 MANUAL OF ELEMENTARY ZOOLOGY folds, which are continuous in front, and behind enclose the blastopore. On either side of the anterior end of the neural folds appears a thickening which becomes divided by a furrow into a gill plate and a sense plate. The neural folds become closer, rise up, bend over, and meet so as to enclose the neural canal, their union taking place first about the middle of their length (Figs. 372, 373, 377, A). Since they enclose the blastopore, the latter comes to lead from the gut to the neural canal and gives rise to a neur
. A manual of elementary zoology . Zoology. 492 MANUAL OF ELEMENTARY ZOOLOGY folds, which are continuous in front, and behind enclose the blastopore. On either side of the anterior end of the neural folds appears a thickening which becomes divided by a furrow into a gill plate and a sense plate. The neural folds become closer, rise up, bend over, and meet so as to enclose the neural canal, their union taking place first about the middle of their length (Figs. 372, 373, 377, A). Since they enclose the blastopore, the latter comes to lead from the gut to the neural canal and gives rise to a neurenteric canal, but this soon disappears. The neural canal separates from the epiblast above it, formed by the outer walls of the neural folds, whose inner sides become the wall of the neural canal. Before the folds have united in front, the open canal between them is enlarged and then divided into three swellings, the rudiments of the fore-, mid-, and hind-brains. It will be seen that in the frog, as in the lancelet, the central nervous system arises by the sinking in and folding off of a strip of the epidermis of the back. This process is found in all Chordata, and is of the highest importance in the drawing of comparisons between them and other animals. During the formation of the central nervous system the body has been elongating and other structures appearing. Below the blastopore, in the area which it occupied before its contrac- tion, there appears a pit known as the proctodeum, and an opening piercing through from this to the gut forms the anus. From anus to blastopore runs a slight groove, the primitive groove. Above it a knob grows out to form the tadpole's tail. Grooves appear on each gill plate marking out the visceral arches, and upon the first two branchial arches branched external gills grow out. Below the head blp. Fig. -An embryo of a later stage. the frog at an., Proctodeum (invagination which will form anus); blp., blastopore; , gill plate; , ne
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectzoology, bookyear1920
