. The chordates. Chordata. 266 Basic Structure of Vertebrates. Fig. 221. Transverse section of the embryonic shield of a rabbit at the stage represented in Fig. 220. The section is taken at the position indicated by the line S-S in Fig. 220. (EC) Ectoderm; (EN) endoderm; (MES) mesoderm; (PG) primi- tive groove of primitive streak. ( X 175.) (After Assheton. Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) of the absence of a large yolk-mass in the mammal. These facts point to the conclusion that the developmental behavior of the reptilian em- bryo ha
. The chordates. Chordata. 266 Basic Structure of Vertebrates. Fig. 221. Transverse section of the embryonic shield of a rabbit at the stage represented in Fig. 220. The section is taken at the position indicated by the line S-S in Fig. 220. (EC) Ectoderm; (EN) endoderm; (MES) mesoderm; (PG) primi- tive groove of primitive streak. ( X 175.) (After Assheton. Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) of the absence of a large yolk-mass in the mammal. These facts point to the conclusion that the developmental behavior of the reptilian em- bryo had become so strongly established in the protoplasm of ancestral reptiles and primitive mammals that it persisted even though the re- duction of yolk had removed the immediate necessity for many of its peculiarities—, the yolk is reduced to a minimum, but a yolk-sac- still persists. The many millions of years of primitive mammalian and reptilian lineage constituted a barrier quite impassable by any possible tendency for reversion to the indefinitely more remote developmental methods of primitive Amphioxus-like chordates. Yet some primitive features persist. There can be little doubt that the blastoporal growth-zone or germ-ring of Amphioxus and amphib- ians and the primitive streak of reptiles are genetically continuous. If, as seems very likely, the axial thickening of the mammalian embryonic shield corresponds to the reptilian primitive streak—with even a mam- malian relic of a blastopore—then the manner of origin of the meso- derm is essentially similar from primitive chordate to mammal. Unquestionably the yolk content of the chordate egg is much more readily subject to evolutionary change than is the developmental mechanism of the germinal protoplasm. That mechanism can be changed, but there is a high degree of inertia about it. The embryo— , the living protoplasmic thing exclusive of inert yolk—is highly conservative. It tends to follow the old methods and
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