DEVELOfMEKI OF THE YOCJNG W1TI3IN THE EGG. 143 



At first the furrow (Fig. 106, I) is very shallow, and a lit- 



Fig. 106. Fig. 107. Fig. 108. 



tie transparent, narrow band appears under it, called the 

 'primitive stripe^ (a.) The walls of the furrow consist of two 

 raised edges formed by a swelling of the germ along both sides 

 of the primitive stripe. Gradually, these walls grow higher, 

 and we perceive that their summits have a tendency to ap- 

 proach each other, as seen in Fig. 107 ; at last they meet 

 and unite completely, so that the furrow is now changed 

 into a closed canal, (Fig. 108, b.) This canal is soon filled 

 with a peculiar liquid, from which the spinal marrow and 

 brain are formed at a later period. 



302. The primitive stripe is gradually obliterated by a 

 peculiar organ of a cartilaginous nature, the dorsal corcl^ 

 formed in the lower wall of the dorsal canal. This is found 

 in the -embryos of all vertebrates, and is the representative 

 of the back-bone. In the mean time, the margin of the 

 germ gradually extends farther and farther over the yolk, so 

 as finally to enclose it entirely, and form another cavity in 

 which the organs of vegetative life are to be developed. 

 Thus the embryo of vertebrates has two cavities, namely, 

 the upper one, which is very small, containing the nervous 

 system, and the lower, which is much larger, for the intes- 

 tines, (161.) 



303. In all classes of the Animal Kingdom, the embryo 

 proper rests upon the yolk, and covers it like a cap. But 

 the direction by which its edges approach each other, and 



from above, it would extend over the yolk in every direction, and the 

 furro\^ at 5, of Fig. 106, would appear as in Fig. 105. 



