. Comparative zoology, structural and systematic : for use in schools and colleges. Zoology; Anatomy, Comparative; Physiology, Comparative. 194 COMPARATIVE ZOOLOGY. spot {cicatricula) on the outside cannot be hardened, even with the most prolonged boiling. The cicatricula, or em- bryo-spot—the part for which all the rest was made—is a thin disk of cellular structure, in which the new life first appears. This was originally a simple cell, but de- velopment has gone some way before the egg is laid. It is always on that side which naturally turns uppermost, for the yolk can turn upon its axis; it
. Comparative zoology, structural and systematic : for use in schools and colleges. Zoology; Anatomy, Comparative; Physiology, Comparative. 194 COMPARATIVE ZOOLOGY. spot {cicatricula) on the outside cannot be hardened, even with the most prolonged boiling. The cicatricula, or em- bryo-spot—the part for which all the rest was made—is a thin disk of cellular structure, in which the new life first appears. This was originally a simple cell, but de- velopment has gone some way before the egg is laid. It is always on that side which naturally turns uppermost, for the yolk can turn upon its axis; it is, therefore, al- ways nearest to the external air and to the Hen's body— two conditions necessary for its development. There is another reason for this polarity of the egg: the lighter and most delicate part of the yolk is collected in its upper part, while the heavy, oily portion remains be- neath. In most eggs the shell and albumen are wanting. When the albumen is present, it is commonly covered by a mem- brane only. In Sharks, the envelope is horny; and in Crocodiles it is calcareous, as in Birds. The egg of the Sponge has no true vitelline membrane, and is not unlike an ordinary amoeboid cell. An egg is, in fact, little more than a very large cell, of which the germinal vesicle is the nucleus. The size of an egg depends mainly upon the quantity of yolk it contains; and to this is proportioned the grade of development which the embryo attains when it leaves the In the eggs fig. 163.—Egg of sponge: of the Star-fishes, Worms, Insects, Mol- * nucleus. ]ugks (except the Cuttle-fishes), many Amphibians, and Mammals, the yolk is very minute and formative, i. e., it is converted into the parts of the future embryo. In the eggs of Lobsters, Crabs, Spiders, Cepha- lopods, Fishes, Reptiles, and Birds, the yolk is large and colored, and consists of two parts — the formative, or. Please note that these images are extracted from scanned page images that may have been
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