. A manual of elementary zoology . Zoology. 49° MANUAL OF ELEMENTARY ZOOLOGY its floor being formed by the large yolk cells over which the lip is growing. The lip is the upper edge of the blastopore, the rest of whose edge is as yet indefinite and represented by the limit of the advancing epiblast all round the egg. All this time the shape of the crescent is changing by its two ends lengthening and curving towards one another till at last they meet to form a circle. By that time the edge of the epiblast has reached this circle all round its circumference, so that all the yolk is covered excep
. A manual of elementary zoology . Zoology. 49° MANUAL OF ELEMENTARY ZOOLOGY its floor being formed by the large yolk cells over which the lip is growing. The lip is the upper edge of the blastopore, the rest of whose edge is as yet indefinite and represented by the limit of the advancing epiblast all round the egg. All this time the shape of the crescent is changing by its two ends lengthening and curving towards one another till at last they meet to form a circle. By that time the edge of the epiblast has reached this circle all round its circumference, so that all the yolk is covered except that within a circular area, the definitive blastopore, bordered by a continuous lip and filled by a. yolk plug consisting of yolk cells which have not yet been covered. The lip continues to grow over the yolk plug, thus narrowing the blastopore. The narrowing, however, takes place not by ingrowth of the lip all round, but by the growing *"â 'â together of the sides of the circle in its hinder part. Where the sides thus coming together meet, there remains a seam in the form of a grooveâthe primitive grooveâunder which lies a band of cellsâthe primitive streakâ in which epiblast, hypoblast, and meso- blast meet and fuse. Finally, the plug is covered and the blastopore is a minute opening at the bottom of a slight de- pression, from which the primitive groove runs backwards. During the Fig. 370.âThe embryo of a later stages of this process an internal frog shortly after the com- movement of the yolk cells has obliter- pletion of gastrulation, ated the blastoccele and enlarged the seen from the right side eriteron, which was at first a mere slit, and somewhat from be- s0 .that ll becomes a spacious cavity, hm(j. which communicates with the ex- terior by a slit between the dorsal «A, Blastopore;«./, neural folds. side of the blastopore lip and the yolk plug. At the end of gastrulation the enteron is a large cavity with a very thick ventral wall composed of Me
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectzoology, bookyear1920
