. Chordate anatomy. Chordata; Anatomy, Comparative. KEPKUDUCTION 57 of a sauropsidan embryo. At the anterior end of this mammalian streak is usually found a small pit or even a perforation extending through the shield into the cavity of the blastodermic vesicle—very suggestive of an abortive blastopore. It is along this mammalian primitive streak, as in the similar sauropsidan structure, that rapid proliferation of cells produces a mesoderm (Fig. 50) which progressively interpolates itself between the already separated ectoderm and endoderm and spreads eventually into all regions of the embryo
. Chordate anatomy. Chordata; Anatomy, Comparative. KEPKUDUCTION 57 of a sauropsidan embryo. At the anterior end of this mammalian streak is usually found a small pit or even a perforation extending through the shield into the cavity of the blastodermic vesicle—very suggestive of an abortive blastopore. It is along this mammalian primitive streak, as in the similar sauropsidan structure, that rapid proliferation of cells produces a mesoderm (Fig. 50) which progressively interpolates itself between the already separated ectoderm and endoderm and spreads eventually into all regions of the embryo. The mesoderm is at first a continuous layer—unsegmented—and devoid of VMES 'en Fig. 50.—Transverse section of the embryonic shield of a rabbit at the stage repre- sented in Fig. 49. The section is taken at the position indicated by the line 5-5 in Fig. 49. EC, ectoderm; EN, endoderm; MES, mesoderm; PG, primitive groove of primitive streak. X175. (After Assheton.) In a rabbit embryo the embryonic shield is established ordinarily by the fifth day of development, the entire blastodermic vesicle then having a diameter of about mm. The early development of the placental mammal presents many perplexing features. It could be expected that the minute egg, unem- barrassed by yolk, would revert to the relatively simple and direct methods of early development which, for the most part, characterize Amphioxus. But it does not. Mammalian stages precisely comparable to the blastula and gastrula of Amphioxus or amphibians cannot be recognized. When it comes to the formation of mesoderm, the laying out of the germ layers, and the early shaping up of the embryo, the behavior of the mammal is closely similar to that of a reptile or bird. This similarity exists in spite of the absence of a large yolk-mass in the mammal. These facts point to the conclusion that the developmental behavior of the reptilian embryo had become so strongly established in the protoplasm of ancestral
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Keywords: ., bookcentury1900, bookpublisherphi, booksubjectanatomycomparative
