. 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. of the germ, to form the dorsal cavity. 304. The embryo in the Articulata, on the contrary, lieswith its back upon the yolk, as seen in the following figure, which represents an embryo of Podurella ;consequently the yolk enters the body onthat side ; and the suture, which in thevertebrates is found on the belly, ie herefound on the back. In the Cephalop
. 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. of the germ, to form the dorsal cavity. 304. The embryo in the Articulata, on the contrary, lieswith its back upon the yolk, as seen in the following figure, which represents an embryo of Podurella ;consequently the yolk enters the body onthat side ; and the suture, which in thevertebrates is found on the belly, ie herefound on the back. In the Cephalopodathe yolk communicates with the lowerside of the body, as in Vertebrates, butthere is no dorsal cavity formed in the other MoUusks, as also in the Worms, there is thispeculiarity, that the whole yolk is changed at the beginninginto the substance of the embryo ; whilst in Vertebrates, andthe higher Articulates and Mollusks, a part of it is reserved,till a later period, to be used for the nourishment of the em-bryo. Among Radiata, the germ is formed around the yolk,and seems to surround the whole of it, from the first.* 305. The development of the embryo of the vertebratedanimals may be best observed in the eggs of fishes. Being. Fiff. 110. * These facts show plainly that the circumstance of embryos arisingfrom the whole or a part of the yolk is of no systematic importance. DEVELOPMENT OF THE IOUNG WITHIN THE EGG. 145 transparent, they do not require to be cut open, and, bysufficient caution, the whole series of embryonic changesmay be observed upon the same individual, and thus the suc-cession in which the organs appear be ascertained with pre-cision ; whereas, if we employ the eggs of birds, which areopaque, we are obliged to sacrifice an egg for each obser-vation. 306. To illustrate these general views as to the develop-ment of the embryo, we will briefly describe the principalphases, as they have been observed in the White-fish of Eu-rope, which belongs to the salm
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