DEVELOPMENT OF THE YOUNG WITHIN THE EGG. 113 



301. The development of this furrow is highly impor- 

 tant in indicating the plan of structure of vertebrated ani- 

 mals in general, as will be shown by the following figures, 

 which represent vertical sections of the embryo at different 

 epochs.* At first the furrow (Fig. 106, I), is very shal- 



Fig. 106. 



Fig. 107. 



Fig. 103. 



low, and a little transparent narrow band appears under 

 it, called the primitive stripe (a). The walls of the 

 furrow consist of two raised edges formed by a swel- 

 ling 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 approach 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, Z>). This canal is soon filled with a peculiar liquid 

 from which the spinal marrow and brain are to be formed. 

 302. The primitive stripe is gradually obliterated by a 

 peculiar organ of a cartilaginous nature, the dorsal cord, 

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

 found in the embryos of all vertebrates, and is the represent- 

 ative 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 cavi- 

 ties, namely, a superior, very small one, for the nervous sys- 

 tem, and an inferior, much larger one, for the intestines. 



* Only the cut edge of the embryo is supposed to be seen, whereas, if 

 viewed from above, it would be seen to extend over the yolk in every direc- 

 tion ; so that the furrow at b, of Fig. 106, would be seen as iu Fig. 105. 



10* 



