422 DEVELOPMENT OF FROG AND OTHER VERTEBRATA 



of fishes may receive a favorable start in life, either by additional 

 food in the yolk of the egg or by greater protection during the 

 developmental stages. From the standpoint of Comparative 

 Embryology, however, the development of amphioxus (c/. Fig. 

 209) is a better introduction to Vertebrate Embryology, since the 

 development of fishes is so greatly modified by the presence of 

 yolk (Fig. 219). The fish embryo develops on the top of this 

 yolk, as a disc-like structure, or blastoderm, in which the parts of 

 the body gradually become apparent along the median axis while 

 the edges of the blastoderm are extending over the surface of the 

 yolk. As the circulatory system is formed, blood vessels extend 

 outward from the body to the yolk mass, which is thus consumed 

 by the embryo. In later stages the connection between embryo 

 and yolk becomes constricted, and the dwindling supply of nutrient 

 material appears in the yolk sac, attached by the yolk stalk to the 

 ventral side of the embryo. Although the exact relationships 

 between embryo and yolk cannot be here described, it may be 

 said that the development of the bodily parts of the fish resembles 

 what occurs in Amphibia, being modified, of course, by the presence 

 of a greater amount of yolk and by differences in adult structures. 

 Development of Reptiles. — The development of a reptile is 

 fairly represented by that of the turtle or the American alligator 

 (Figs. 220 and 221). The familiar turtles of fresh water, like the 

 genus Chrysemys, produce eggs resembling those of a bird, except 

 that they are encased in a leathery covering instead of a shell of car- 

 bonate of hme. Spermatozoa are introduced into the anal open- 

 ing of the female during sexual union and thence find their way 

 to the anterior end of the oviduct where fertilization occurs. In 

 the oviduct the '' white " and " shell " are secreted about the 

 '' yolk," which is the part arising from the ovary and is therefore 

 comparable with the ovum of the frog. At the time of laying, 

 the female comes out upon the land and excavates a cavity, some- 

 times in hard ground at a considerable distance from the shore. 

 Here a dozen or more eggs are laid, several inches below the sur- 

 face, and, after being covered loosely with soil, are left to be incu- 

 bated by the heat of the sun. The young emerge as miniature 

 adults as do chicks from the eggs of a hen (Fig. 220 G), and 

 immediately make for the water, where they may sometimes 

 be seen in large numbers. The Florida alligator lays its eggs 

 upon the land, in a rough nest constructed of flags and other 



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