. Embryology. Embryology; Embryology; genealogy. 32 EMBRYOLOGY ECTODERM. CORTEX ENDOPLASM ENDODERM LOCALIZATION MESENCHYME CORTEX Fig. 13. Hypothetical lo- calization of the factors for ectoderm, endoderm, and mesoderm (mesenchyme) in the cortex of the sea urchin egg. "Ectoderm" stands for substances or factors which in later development are respon- sible for the formation of the ectoderm of the embryo. Sim- ilarly with "endoderm" and "; Where in the egg itself—the unfertilized egg-—might this organization be? It cannot be in the endoplasm, for this reg
. Embryology. Embryology; Embryology; genealogy. 32 EMBRYOLOGY ECTODERM. CORTEX ENDOPLASM ENDODERM LOCALIZATION MESENCHYME CORTEX Fig. 13. Hypothetical lo- calization of the factors for ectoderm, endoderm, and mesoderm (mesenchyme) in the cortex of the sea urchin egg. "Ectoderm" stands for substances or factors which in later development are respon- sible for the formation of the ectoderm of the embryo. Sim- ilarly with "endoderm" and "; Where in the egg itself—the unfertilized egg-—might this organization be? It cannot be in the endoplasm, for this region is full of granules and these granules can be shifted. As we have seen, this endoplasm can be dis- turbed by the centrifuging process and the egg still develops normally. The only other possible location of organization is in the outer cortex. This has a much higher consistency or viscosity; it is a relatively stiff substance. In the process of centrifuging, then, it would be this thin outer layer which would remain undisturbed. Therefore it may contain the organization of the egg into the substances which give rise to the three germ layers (Fig. 13). Thus we picture the outer layer of the animal hemisphere as containing the factors for ectoderm. The ectodermal factors extend a little below the equator, forming a sort of shell. Next there is a thin shell of protoplasm which contains the factors for the formation of endoderm. And finally at the vegetal pole itself there is a layer of protoplasm in the cortex which causes mesoderm to form. The labile nature of organization Now, how stable is the organization into the protoplasm of the three germ layers? We pointed out that if the egg is cut in two through the equator the two halves can be fertilized and will develop into deficient larvae ( Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may no
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