516 



EMBRYOLOGY OF THE CHICK 



THE SKELETAL STRUCTURE 



On the fourth day the mesoderm surrounding the brain has increased 

 and begins to show slight traces of the skull formation toward the an- 

 terior portion of the head, and extending posteriorly. The fronto-nasal 

 process has already been discussed as well as the formation of the upper 

 and lower jaws. 



The beginnings of the vertebral column are also in evidence, though 

 only to a slight extent. Nevertheless, it is well at this point to sum- 

 marize what will occur, so that future changes will be understandable. 



During the fourth day the somites have increased from about thirty 

 to forty, each somite showing a more or less distinctive division into an 

 outward lying muscle plate, and an inner region which is to form the 

 vertebral column. It is from these inner portions that processes of 

 mesoderm are sent out by both dorsal and ventral regions to the neural 

 canal, as well as below the notochord, until these structures are com- 



i. 



Occip.- 

 SkleroK 



Myocoel 



Vert 3 



Fig. 305. 



I, Redividing of the spinal segments. On the left side of the cut the sclerotomes 

 and myotomes are seen in their original state. On the right they are seen in their 

 final state. The cephalic portions are dotted and the caudal portions hatched. The 

 arrows show the line of demarcation between head and neck. II, Ventral view of 

 spinal column to show redivided parts of each vertebra. (From Corning, after 

 Kollmann.) 



pletely surrounded by mesoderm. By the end of the fourth day these 

 processes have become thickened, and are often called the membranous 

 vertebral column. The membranous vertebral column is still segmented, 

 each segment corresponding to the original somite from which it sprang. 



On the fifth day these lines of segmentation disappear in the meso- 

 derm which becomes continuous in its surrounding of the neural canal 

 and notochord, though the muscle plates retain their segmentation. 



On the fifth day also, the mesoderm lying in immediate contact 

 with the notochord becomes cartilaginous, to form a cartilaginous sheath 



