. A text-book of invertebrate morphology. Invertebrates. Pig. 36.—Diagram Illustrating the Formation of the Diploblastic Stage (A) by Delamination, (5) by Immigration. mass, results. This is known as the parenchymella or sterrvla. Later a cavity appears in the centre of the solid mass, whose cells gradually are pushed towards the periphery, where they form eventually a single layer, the endoderm. Finally a blas- topore is formed and the embryo bfecomes a gastrula. It does not seem easy to bring the delamination and invagination methods of gastrulation into direct relation with each other, or t
. A text-book of invertebrate morphology. Invertebrates. Pig. 36.—Diagram Illustrating the Formation of the Diploblastic Stage (A) by Delamination, (5) by Immigration. mass, results. This is known as the parenchymella or sterrvla. Later a cavity appears in the centre of the solid mass, whose cells gradually are pushed towards the periphery, where they form eventually a single layer, the endoderm. Finally a blas- topore is formed and the embryo bfecomes a gastrula. It does not seem easy to bring the delamination and invagination methods of gastrulation into direct relation with each other, or to derive one from the other, but it is probable that both must be referred back to the immigration method. In typical cases of immigration the cells which migrate are situated irregularly at any part of the blastula, but frequently, especially in free-swimming blastulas, the migrating cells are all located nt the posterior extremity. If in such cases of polar immigration the migrat- ing cells were to pass into the blastocoel en masse instead of individually, invagination would result. On the other hand, if a considerable amount of yolk were present in all the cells of a blastula, it might happen that, in- stead of migrating, the cell might undergo division, cutting off the yolk- containing protoplasm from the yolkless, delamination thus taking 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 not perfectly resemble the original McMurrich, J. Playfair (James Playfair), 1859-1939. New York, H. Holt and Company
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