. The biology of the amphibia. Amphibians. 22 THE BIOLOGY OF THE AMPHIBIA laden cells of the opposite pole. A continuation of this process causes the cells of the first region to tend to grow over those of the second. The gray crescent region takes the lead in cell proliferation and becomes the dorsal lip of the blastopore, growing as a crescentic fold over a section approximately 100 degrees of the egg. The slitlike cavity between lip and infolded yolk cells represents the archenteron, or rudiment of the gut. This eventually either opens at its anterior end into the blastocoel or obliterates
. The biology of the amphibia. Amphibians. 22 THE BIOLOGY OF THE AMPHIBIA laden cells of the opposite pole. A continuation of this process causes the cells of the first region to tend to grow over those of the second. The gray crescent region takes the lead in cell proliferation and becomes the dorsal lip of the blastopore, growing as a crescentic fold over a section approximately 100 degrees of the egg. The slitlike cavity between lip and infolded yolk cells represents the archenteron, or rudiment of the gut. This eventually either opens at its anterior end into the blastocoel or obliterates it by crowding. Since the overgrowth of the cells of the dorsal hemisphere extends completely around the egg, the blastopore becomes a gradually diminishing circular fold engulfing. Fig. 10.—Blastopore of a salamander and caecilian compared. In salamanders and most frogs the overgrowth of cells of the dorsal hemisphere during gastrula- tion extends completely around the egg; in caecilians the blastodisc or overgrowth forms a circular blastopore while the yolk hemisphere is still uncovered. A. Late blastopore of Cryptobranchus {after Smith). B and C. Two stages in the formation of the blastopore of Ichthyophis {after the Sarasins). , blastodisc; , blastopore; , neural plate. the more slowly dividing yolk cells. In some plethodontid salamanders, the circle remains incomplete ventrally and the blastopore takes the form of an inverted crescent (Humphrey, 1928). The embryo-forming materials of the gray crescent are at first broadly distributed as a ring or crescent about the circumference of the egg. They are brought together not only by overgrowth during gastrulation but also by concrescence of the two halves of the gray crescent in the midline. Hence the point of concrescence, namely the dorsal lip of the blastopore, comes to have a more important role in organ formation than the ventral lip. In frogs and salamanders the whole of the yolk hemisphere is covered over a
Size: 3031px × 824px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookpublishernewyorkmcgr, booksubjectamphibians
