THE SOMITES OF THE CHICK 



79 



teenth somite of a slightly older embryo (fig. 12). The growth 

 of the segments is shown in the longitudinal sections as well as in 

 the transverse sections, as will be seen from the following table 

 of measurements. 



In embryos of thirty segments, all of the somites are from 139 

 to 156 microns long. 



Furthermore the later somites do not recapitulate the develop- 

 ment of the earlier somites; on the contrary, they merely omit 

 the initial phases of the development of the earlier somites. Thus 

 the core gradually forms in the second somite whereas that of the 

 tenth and later somites is present from the first. In the same way 

 the upper myotomic groove appears in the second somite after 

 the formation of the ninth segment, but in the twenty-fifth and 

 thirtieth somites it appears almost at once. So also the neph- 

 rotome is in a more advanced stage of development in the newlj 

 formed posterior segments than in the anterior segments at the 

 time of their origin. 



