THE EMBRYO. 



287 



and is converted from an open groove to a closed cylinder 

 by its margins growing up and meeting together above it. In 

 a similar way its contents likewise become cylindrical; the 

 tube thus formed being converted in its lower part into the 

 spinal cord; while in the cerebral part, which at this early 

 stage is little less than half the 

 length of the whole embryo, it is 

 swollen into three successive vesi- 

 cles. The first or foremost of 

 these cerebral vesicles is that from 

 which the third ventrical of the 

 brain, with the hemisphere-vesi- 

 cles coming off from it, is deve- 

 loped; the second is that from 

 which the aqueduct of Sylvius, 

 with the corpora quadrigemina and 

 other parts surrounding it, is 

 formed; and the third is the part 

 from which the cerebellum, pons 

 Yarolii, and medulla oblongata 

 take origin. 



The chorda dorsalis or noto- 

 chord. runs down the centre of 

 the middle layer of the embryo. 

 It is a purely cellular structure, 

 and continues so as long as it 

 exists, but in most animals it is 

 not permanent. In sturgeons, 

 lampreys, and some other fishes, it 

 continues through life, being de- 

 veloped into a thick column of 

 large, distinctly-walled cells, with 

 a thick fibrous sheath round about, 

 which serves instead of a chain of 

 bodies of vertebrae. In other ver- 

 tebrate animals, the bodies of the 

 vertebrae make their first appear- 

 ance round the sheath of the chorda dorsalis, and constrict 

 that structure so as to leave a bead-like dilatation in the 

 position of each intervertebral disc; ultimately, however, in 



Fig. 152. EMBRYO CHICK, 

 about two days old; under 

 surface, a, b, c, First, second, 

 and third cerebral vesicles; 



d, primary optic vesicle; 



e, rudiment of heart; /, 

 fold at which the cephalic 

 plate is continuous with 

 the cephalic hood, and the 

 yelk sac continuous with 

 the cul-de-sac from which 

 the pharynx is formed ; g, 

 primordial vertebrae ; h, 

 unclosed part of the primi- 

 tive groove. 



