GENERAL HISTORY OF THE EMBRYO. 



367 



brain and of the sense organs, more especially the eye, in the 

 rabbit. 



The first trace of the neural groove appears in the chick 

 embryo about the eighteenth hour of incubation, and in the 

 rabbit embryo early on the eighth day. Starting from this 

 stage, the rate of development is approximately the same in the 

 two embryos ; the twelfth day rabbit embryo corresponding to 

 the chick embryo about the middle of 

 the sixth day. 



By the twentieth day the rabbit 

 embryo has attained the shape and size 

 shown in Fig. 149 ; in grade of develop- 

 ment, and also in actual dimensions, 

 it corresponds very closely to a chick 

 embryo of the twelfth day . 



The young rabbit is born on the 

 thirtieth day, i.e. about twenty-two 

 days from the time of first appearance 

 of the neural groove, the earliest formed 

 organ in the body. The chick is 

 hatched on the twenty-first day of 

 incubation, or rather more than twenty 

 days from the same starting-point. 

 The young rabbit at birth is of con- 

 siderably greater bulk than the chick 

 on hatching, but is in a far more helpless condition ; the eyelids 

 are still united together, and the young animal is quite incapable 

 of looking after itself, and would perish but for the supply of 

 milk afforded it by the mother. 



Fig. 149.— A Rabbit Em- 

 bryo of the twentieth- 

 day, seen from the right 

 side. The rows of spots, 

 round the nose and 

 above the eye, and the 

 single large spot below 

 the eye, represent hair 

 follicles, the last-men- 

 tioned one being of 

 especial size, x 1. 



2. The Yolk-sac. 



The yolk-sac is the extra-embryonic portion of the blasto- 

 dermic vesicle ; i.e. the part which is left after the embryo is 

 constricted off by the head, tail, and side folds. 



The yolk-sac (Figs. 146, 147, 148, ys), though corresponding 

 in its mode of formation, and in its relations to the embryo, with 

 the yolk-sac of the chick embryo (c/. Fig. 100), differs from this 

 latter in one very important respect. The yolk-sac of the bird 

 is filled with food matter for the nutrition of the embryo, and 

 affords the supply of nourishment at the expense of which the 



