. A manual of zoology. PHYLUM CHORDATA 3*5 through the common aperture, into one or other of the ovi- ducts, where fertilisation (p. 393) occurs. As it passes into the dilated portion of the oviduct, the oosperm (p. 60) of Scyllium becomes surrounded by a horn-like egg-shell (Fig. 229), secreted by the shell-gland, and having the form of a pillow-case produced at each of its four corners into a long tendril-like process. The eggs are laid among seaweed, to which they become attached by their tendrils. In some other dogfishes (Acanihias, Mus- telus) a mere vestige of the egg-shell is formed, an
. A manual of zoology. PHYLUM CHORDATA 3*5 through the common aperture, into one or other of the ovi- ducts, where fertilisation (p. 393) occurs. As it passes into the dilated portion of the oviduct, the oosperm (p. 60) of Scyllium becomes surrounded by a horn-like egg-shell (Fig. 229), secreted by the shell-gland, and having the form of a pillow-case produced at each of its four corners into a long tendril-like process. The eggs are laid among seaweed, to which they become attached by their tendrils. In some other dogfishes (Acanihias, Mus- telus) a mere vestige of the egg-shell is formed, and the eggs undergo the whole of their development in the oviducts, the young being eventu- ally born alive with the form and proportions of the adult. The great size of the egg is due to the immense quantity of yolk which it contains; its protoplasm is almost entirely aggregated at one pole in the form of a small disc. When segmentation of the oosperm takes place, it affects the protoplasm alone, the inactive yolk taking no part in the process. The disc of protoplasm divides to form a little heap of cells, the blastoderm, situ- ated at one pole of the undivided sphere of yolk. The blastoderm subsequently spreads out as a sheet of cells over the yolk which it ultimately completely encloses. While this extension of the blastoderm is taking place, its middle part becomes raised up into a ridge-like thick- 2 c. Fig. 229. —Dogfish, egg-case. (After Dean ). 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 Parker, T. Jeffery (Thomas Jeffery), 1850-1897; Haswell, William A. (William Aitcheson), 1854-1925. New York, The Macmillan Company; London, Macmillan & Co. ,Ltd.
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