. Comparative anatomy. Anatomy, Comparative. 76 COMPARATIVE ANATOMY -o The mesoderm of Amphioxus is segmented at the time of its detach- ment from the primary gastrular layers and some of the more anterior segments are hollow. The amphibian mesoderm is primarily unseg- mented and solid. In view of the fact that it later acquires segmentation and hollowness, these initial differences are outweighed by the essential similarity in the relations to the blastoporal region. In Reptiles and Birds. In reptiles and birds the formation of an endoderm is initiated by a small invagination or infolding at
. Comparative anatomy. Anatomy, Comparative. 76 COMPARATIVE ANATOMY -o The mesoderm of Amphioxus is segmented at the time of its detach- ment from the primary gastrular layers and some of the more anterior segments are hollow. The amphibian mesoderm is primarily unseg- mented and solid. In view of the fact that it later acquires segmentation and hollowness, these initial differences are outweighed by the essential similarity in the relations to the blastoporal region. In Reptiles and Birds. In reptiles and birds the formation of an endoderm is initiated by a small invagination or infolding at the posterior edge of the early blastoderm (see page 69). The abortive blastopore thus produced exhibits the usual feature of a blastopore in that, in terms of germ layers, it is an indifferent region where ectoderm and endoderm merge together without sharp of demarcation (Fig. 45). Following gastrula- tion the blastodermal layers continue to -PS Fig. 49.—Surface view blastoderm of Chick after 15 hours incubation. C, "anterior cres- cent," occasioned by an irregular Spread rapidly over the surface of the yolk. fold of underlying endoderm; M. j^ go doing, the growth posteriorly causes region occupied by mesoderm; , . O, area opaca whose opacity is the somewhat thickened region of the caused by adherence of yolk to blastoporal rim to become drawn out into a the blastoderm; P, area pellucida long Streak, the "primitive streak," lying in the median line of the blastoderm (Fig. 49). Along the whole extent of this modi- fied blastoporal region the ectoderm and endoderm merge without sharp demarcation just as they did in the earlier blastoporal wall (Fig. 50). This primitive streak is the primary seat of mesoderm formation. Rapid cell proliferation within the substance of the thickened "streak" —transparent in absence of adher- ing yolk (see Fig. 45); PS, primitive streak. X14. (After Duval, Atlas d'Embryologie.). y--^:^~^t Fig. 50.—Section transver
Size: 3710px × 674px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookpublisherphi, booksubjectanatomycomparative
