206 THE FOURTH DAY. [CHAP. 



site the intervals between the vertebrae. For this reason, when 

 the vertebras became formed, their centres were opposite not the 

 middle of the muscle- plates but the inter-muscular septa. 



These considerations fully explain the characters of the 

 secondary segmentation of the vertebral column. On the other 

 hand the primary segmentation of the vertebral rudiments is 

 clearly a remnant of a condition when no vertebral bodies were 

 present ; and has no greater morphological significance than the 

 fact that the cells of the vertebrae were derived from the seg- 

 mented muscle-plates, and then became fused into a continuous 

 sheath around the notochord and nervous axis ; till finally they 

 became in still higher forms differentiated into vertebrae and 

 their arches. 



By these changes this remarkable result is brought 

 about, that each permanent vertebra is formed out of 

 portions of two consecutive mesoblastic somites. Thus, 

 for instance, the tenth permanent vertebra is formed 

 out of the hind portion of the tenth somite, and the 

 front portion of the eleventh somite. 



The new segmentation is associated with or rather is 

 caused by histological changes. At the time when 

 the fusion takes place, the mesoblast, which in the form 

 of processes from the somites surrounds and invests 

 the notochord, has not only increased in mass but also 

 has become cartilaginous, so that, as Gegenbaur 1 points 

 out, there is present for a short period on the fifth day 

 a continuous and unsegmented cartilaginous investment 

 of the notochord. 



This cartilaginous tube does not however long re- 

 main uniform. At a series of points corresponding in 

 number to the original somites it becomes connected 



1 Untersuchung zur vergleichenden Anatomic der Wirbelsaule lei 

 Amphibien und Reptilien, Leipzig, 1862. 



