. Text-book of embryology. Embryology. INTEODUCTION 27. free protoplasm (centrolecithal) (Fig. 11). The result of this latter dis- tribution is that a skin of cells is formed over an inert mass of yolk. But the clogging influence of yolk extends iar beyond ,^=^^^=\ /9^ the first stages of develop- ment. The course of development can indeed be roughly divided into three stages:—(1) In the first the zygote becomes divided into a number of em- bryonic cells or blastomeres; this stage is called segmen- tation; (2) in the second these cells are arranged so as to form the primary organs, the so-call
. Text-book of embryology. Embryology. INTEODUCTION 27. free protoplasm (centrolecithal) (Fig. 11). The result of this latter dis- tribution is that a skin of cells is formed over an inert mass of yolk. But the clogging influence of yolk extends iar beyond ,^=^^^=\ /9^ the first stages of develop- ment. The course of development can indeed be roughly divided into three stages:—(1) In the first the zygote becomes divided into a number of em- bryonic cells or blastomeres; this stage is called segmen- tation; (2) in the second these cells are arranged so as to form the primary organs, the so-called germ layers, the skin, and the lining of the gut and of the body cavity; this stage is called the forma- tion of the layers; and (3) in the third stage these layers are modified into the larval or permanent organs; this last stage is called organogeny. Eggs with Little or no yolk are termed alecithal (Fig. 12). If yolk in the form of refringent globules should be totally absent, reserve stuffs in the shape of masses of chromatin are scattered about through the cytoplasm. In such eggs the building up of organs out of the first cells, or blastomeres which result from division, takes place by the simplest processes of unequally rapid growth of different parts, and of folding. iSTow in the folding of a layer of cells it is essential that the radius of curvature should bear such a relation to the size of the individual cell that the latter should not be deformed. When the layer consists of a few large yolky cells, folding becomes impossible and is replaced by proliferation of new cells at one point in the layer. Food, as we have seen, is usually supplied to the immature ovum by the sacrifice of the less fortunate oogonia or immature ova. In the case of the common polyp. Hydra, the im- mature egg comes at this stage to resemble an Amoeba. But in one FiQ. 11.—Unripe egg of Limulus polyphemiis. (After Munson.) An example of a centro- lecitlial egg. ch, chorion; , nucleol
Size: 1576px × 1585px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, bookpubli, booksubjectembryology
