VII.J THE NOTOCHOED. 209 



on the sixth day the vacuoles in each cell have so much 

 increased at the expense of the protoplasm that only a 

 very thin layer of the latter is left at the circumference 

 of the cell, at one part of which, where there is gene- 

 rally more protoplasm than elsewhere, the starved re- 

 mains of a nucleus may generally be detected. Thus 

 the whole notochord becomes transformed into a spongy 

 reticulum, the meshes of which correspond to the vacu- 

 oles of the cells and the septa to the remains of their 

 cell-walls. 



The notochord is on the sixth day at the maximum 

 of its development, the change which it henceforward 

 undergoes being of a retrograde character. 



From the seventh day onward, it is at various points 

 encroached upon by its investment. Constrictions are 

 thus produced which first make their appearance in the 

 intervertebral portions of the sacral region. In the cer- 

 vical region, according to Gegenbaur, the intervertebral 

 portions are not constricted till the ninth day, though in 

 the vertebral portions of the lower cervical vertebrae con- 

 strictions are visible as early as the seventh day. By 

 the ninth and tenth days, however, all the interverte- 

 bral portions have become distinctly constricted, and at 

 the same time in each vertebral portion there have also 

 appeared two constrictions giving rise to a central and 

 to two terminal enlargements. In the space therefore 

 corresponding to each vertebra and its appropriate in- 

 tervertebral portion, there are in all three constrictions 

 and three enlargements. 



On the twelfth day the ossification of the bodies 

 of the vertebrae commences. The first vertebra to ossify 

 is the second or third cervical, and the ossification gradu- 

 P. & B. 14 



