VI.] THE NOTOCHORD. 157 



chord of the sixth day might be supposed to have such a structure, but the study 

 of its development and a careful examination of its structure proves that this is 

 not a correct account. 



According to the measurements of Miiller (loc. cit.} the diameter of the 

 notochord on the third day is o'op mm. and that of the central cells o'Ois 

 0*018. On the fourth day the notochord is 0*16 mm. in diameter and its com- 

 ponent cells are also larger. On the sixth day its diameter is at the maximum, 

 and reaches 0*2 inm. The central cells measure 0*02 mm. 



From these measurements it will be seen that the vacuolation of the cells 

 of the notochord is accompanied by a rapid growth both in the size of the cells 

 and in the diameter of the notochord itself. 



14. The notochord is on the sixth day at the maximum of 

 its development, the change which it henceforward under- 

 goes 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 interverte- 

 bral portions of the sacral region. In the cervical region, 

 according to Gegenbaur, the intervertebral portions are not 

 constricted till the ninth day, though as early as the seventh 

 day constrictions are visible in the vertebral portions of 

 the lower cervical vertebrae. By the ninth and tenth days, 

 however, all the intervertebral 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 

 intervertebral portion, there are in all four constrictions and 

 three enlargements. 



On the twelfth day the ossification of the bodies com- 

 mences. At that time, according to Schwarck (loc. cit.}, the 

 cartilaginous bodies of the vertebras are composed of an inner 

 layer in which the cells form lines radiating from the noto- 

 chord, and an outer layer somewhat sharply separated from 

 the inner one. In the inner layer, immediately around the 

 notochord, ossification first commences. 



Gegenbaur (loc. cit. p. 67) considers that this layer in which ossification 

 commences corresponds to the primordial body of the vertebrae in amphibians. 

 Schwarck is doubtful whether it corresponds to his inner layer of cartilage in 

 the first stage. 



In rare cases ossification first commences as a deposit on the exterior of the 

 vertebrae. 



The first vertebra to ossify is the second or third cervical, 

 and the ossification gradually extends backwards. It does 



