634 



REPRODUCTION. 



of the medullary groove still extends a certain distance beyond them, 

 and they are also absent from the region of the head. 



The subsequent history of the protovertebrae consists in their trans- 

 formation into other tissues and their final disappearance as distinct 

 organs. Their upper and outer portions are mainly converted into the 

 voluntary muscles covering the spinal column, while their inferior and 

 inner portions supply the material for the bodies of the vertebrae, the 

 vertebral arches, and the intervertebral ligaments. During this process 



FIG. 188. 



RUDIMENTARY EMBRYO 

 OF THE CHICK, at the 

 thirty-sixth hour of in- 

 cubation. Dp. Dorsal 

 plates, cephalic region. 

 Mg. Medullary groove. 

 Pv. Anterior pair of 

 protovertebrse. ( K61- 

 liker.) 



EMBRYO OF CHICK, about the fortieth hour of in- 

 cubation. Ce. Cephalic extremity. Pv. Proto- 

 vertebrse. Dp. Dorsal plates, still widely sep- 

 arated in the caudal region. Pr. Primitive 

 trace. (Kolliker.) 



they become, for the most part, fused with each other in the longitu- 

 dinal direction, and by the end of the fifth day the divisions between 

 them are no longer visible. 



The chorda dorsalis, already mentioned (page 620), is a slender 

 longitudinal cylinder, rather less than 0.1 millimetre in diameter, situ- 

 ated in the median line, between the ectoderm and entoderm, imme- 

 diately beneath the medullary canal. It first appears, from the twentieth 

 to the twenty-fourth hour of incubation, at the posterior part of the 

 medullary groove. It thence extends forward, during the second day, 

 to a point corresponding with the middle region of the head. It is 



