. Text book of zoology. Zoology. IV. Embryology. 45- The cells composing this mass are destined to degenerate later on^ and to serve as food for tlie rest (food yolk). The invagination too, occurs in a somewhat peculiar way: first of all, a fold is formed on one side of the blastula (Fig. 34, 2), and gradually surrounds the ovum; then it grows down over the lower portion of the egg, so that this comes to project into the Fig. 34. —Formation of the gastrula in Amphibia, diagrammatic, longitudinal section. 1 Blastula. 2 The invagination has begun at i (the corresponding place in 1
. Text book of zoology. Zoology. IV. Embryology. 45- The cells composing this mass are destined to degenerate later on^ and to serve as food for tlie rest (food yolk). The invagination too, occurs in a somewhat peculiar way: first of all, a fold is formed on one side of the blastula (Fig. 34, 2), and gradually surrounds the ovum; then it grows down over the lower portion of the egg, so that this comes to project into the Fig. 34. —Formation of the gastrula in Amphibia, diagrammatic, longitudinal section. 1 Blastula. 2 The invagination has begun at i (the corresponding place in 1 is indicated by an arrow) ; the invagination is in the form of a furrow, but does not yet surround the egg. 3 The invagination is proceeding. 4 Perfect gastrula; the archenteron is almost filled with a projecting part of the hypoblast, which is later dissolved and absorbed by the embryo, eh epiblast (light), en hypoblast (shaded), g gastrula mouth, h segmentation cavity, i invagination furrow, w archenteron.—Orig. Tliis mode of gastrula formation is readily derived from the typical one; and indeed, simply results from the excessive thickening of the hypoblast cells. If to the blastula figured in Fig. 29 4, there vcere added a large mass of hypoblast cells, which took no active part in invagination, this would occur as in Pig. 34. 5. This is the kind of development which occurs in many Pish, in Reptiles and in Birds. The egg-cell contains a large amount of yolk, practically all the protoplasm is accumulated at one pole of the egg, and it alone segments, while the greater part of the cell remains- unsegmented; this is known as partial 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 Boas, J. E. V. (Johan Erik Vesti), 1855-1935; Boas, J. E. V. (Johan Erik Vesti), 1855-1935. Lehrbuch der Zoolo
Size: 1640px × 1523px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1896
