288 



ANIMAL PHYSIOLOGY. 



the higher vertebrata the whole chorda 'dorsalis completely 

 disappears, even the intervertebral discs being developments 

 rather of its sheath than of its proper substance. The chorda 

 dorsalis has been traced in young mammals into the region 

 of the sphenoid bone, where it ends in a point imbedded 

 in the cartilage of the base of the skull. 



210. On each side of the chorda dorsalis, in the early 

 embryo, the middle layer, except in the head, is divided into 

 a part near the middle line called the dorsal plate, and a part 

 beyond termed the ventral plate. 



Fig. 153. TRANSVERSE SECTION OP CHICK of two days incubation. 

 a, Spinal cord; &, central canal of cord; c, outer layer of embryo, 

 forming the cuticle ; d, primordial vertebra ; e, chorda dorsalis ; 

 /, inner layer of embryo, forming intestinal epithelium ; g, ven- 

 tral plate pushing towards the middle line ; h, *, outer and inner 

 division of the same ; k, outer layer of blastoderm, from which 

 the amnion and inner part of the chorion are formed ; I, inner 

 layer of blastoderm which surrounds the yelk. 



The dorsal plates soon exhibit a distinct segmentation, 

 a series of blocks of dense tissue, the primordial vertebrae, 

 making their appearance on each side of the primordial 

 groove, beginning behind the head, and increasing in num- 

 bers backwards. Each of these so-called primordial vertebras 

 contains superficially the rudiment of a segment of the 

 muscular wall of the body, and, beneath that, the commence- 

 ment of a spinal nerve, which afterwards pushes inwards to 

 join the spinal cord; also between the successive pairs of 

 spinal nerves appear the rudiments of the skeleton, namely, 

 the vertebral and costal arches. The vertebral and costal 

 parts of each segment are indivisible at first, but afterwards, 

 as the common cartilaginous mass grows outwards, it becomes 

 divided, in the thoracic region, into vertebra, rib, costal car- 

 tilage, and part of the sternum, and folds round to meet its 

 fellow of the opposite side. 



