DEVELOPMENT OF THE YOUXG WITHIN THE EGG. 281 



row, marking the position which the future back-bone is to 



occupy (fig- 291). 



458. The development of this furrow is highly impor- 

 tant, as indicating the plan of structure of vertebrated animals 

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

 represent vertical sections of the embryo at different epochs.* 



Fig. 292. Fig. 293. Fig. 294. 



At first the furrow (fig. 292, b) is very shallow, 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 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 approach 

 each other, as seen in fig. 293 ; at last they meet and unite 

 completely, so that the furrow is now changed into a closed 

 canal (fig. 294, 6). This canal is soon filled with a peculiar 

 liquid, from which the spinal cord and brain are formed at a 

 later period. 



459. 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 vertebrata, 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 the vertebrata has two cavities, namely, the upper 

 one, which is very small, containing the nervous system, and 

 the lower, which is much larger, for the intestines ( 226). 



460. In all classes of the animal kingdom, the embryo 

 proper rests upon the yolk, andcoversitlike a cap. Butthe direc- 



* In these figures the egg is supposed to be cut down through the 

 middle, so that only the cut edge of the embryo is seen ; whereas, if 

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

 and the furrow at b, of fig. 292, would be seen as in fig. 291. 



