. Principles of zoölogy : touching the structure, development, distribution, and natural arrangement of the races of animals, living and extinct with numerous illustrations : Part 1, Comparative physiology : for the use of schools and colleges. Fig. 117. Fig. 118. Fig. 119. each other until they unite and form a canal, ^Fig. 117, b,) DLVELOPMENT OF THE YOUNG WITHIN THE EGG. 147 as has been before shown, (Fig. 107.) At the same /me,an enlargement at one end of the furrow is observed. Thisis the rudiment of the head, (Fig. 118,) in which may soonbe distinguished traces of the three divisions of
. Principles of zoölogy : touching the structure, development, distribution, and natural arrangement of the races of animals, living and extinct with numerous illustrations : Part 1, Comparative physiology : for the use of schools and colleges. Fig. 117. Fig. 118. Fig. 119. each other until they unite and form a canal, ^Fig. 117, b,) DLVELOPMENT OF THE YOUNG WITHIN THE EGG. 147 as has been before shown, (Fig. 107.) At the same /me,an enlargement at one end of the furrow is observed. Thisis the rudiment of the head, (Fig. 118,) in which may soonbe distinguished traces of the three divisions of the brain,(Fig. 119,) corresponding to the senses of sight, (m,) hear-ing, (e,) and smell, (p.) 310. Towards the thirteenth day, we see a transparent,cartilaginous cord, in the place afterwards occupied by theback-bone, composed of large cells, on which transverse. Fig. 120. Fig. 121. Fig. 122. divisions are successively forming, (Figs. 120, 121, c.) Thisis the dorsal cord, a part of which, as we have before seen, iscommon to all embryos of vertebrated animals. It alwaysprecedes the formation of the back-bone ; and in somefishes, as the sturgeon, this cartilaginous or embryonic stateis permanent through life, and no true back-bone is everformed. Soon after, the first rudiments of the eye appearin the form of a fold in the external membrane of the germ,in which the crystalline lens (Fig. 121, x) is afterwardsformed. At the same time we see, at the posterior partof the head, an elliptical vesicle, which is the rudiment of theear. At this period, the distinction between the upper andihe lower layer of the germ is best traced ; all the changesmentioned above appertaining to the upper layer. 811. After the seventeenth day, the lower layer dividesinto two sheets, the inferior of which becomes the intestine 148 EMBRYOLOGY. The heart shows itself about
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