56 LEONARD W. WILLIAMS 



lafel of the following somite, to which it becomes attached. Since 

 each arch retains its connection with the dorsal lamella of its own 

 somite, it is now attached to the dorsal lamellae of two adjacent 

 somites. There now forms in the perichordal sheath at the cen- 

 ter of each segment, a conspicuous condensation which is sepa- 

 rated from the condensations of the adjacent segments by light 

 transverse zones, which Remak believed were clefts. These 

 condensations, however, do not correspond with the vertebral 

 centra and, to distinguish them from the latter, Remak called 

 them the primitive vertebral centra. 



During the fifth and sixth days, a striking change occurs in the 

 perichordal sheath. This consists in the apparent resegmenta- 

 tion, Neue Gliederung, of this sheath, by which the definitive 

 vertebral centra are formed. Since the primitive centra arise from 

 the whole medial ventral edge of the somite, and the vertebral 

 arch comes from the posterior part of each somite, the arch is con- 

 sequently attached to the posterior part of the primitive centrum. 

 The spinal nerve and ganglion occupy a space between two verte- 

 bral arches which corresponds with the anterior part of the prim- 

 itive centrum. However, in a later stage the relation between 

 these structures is reversed: the arch is now attached to the an- 

 terior end of the centrum and the ganglion lies in a space which 

 corresponds with the posterior end of the centrum. This change 

 in the position of the nerve in relation to the vertebral arch is the 

 basis of Remak's assertion that there is a resegmentation of the 

 vertebral centra, or more accurately of the perichordal sheath 

 (Wirbelkorpersdule.) It will be remembered that Remak worked 

 with whole or dissected embryos, consequently it is not surpris- 

 ing that he mistook the primary perichordal condensations which 

 form the intervertebral ligaments for the vertebral centra. This 

 error was corrected by Gegenbaur ('62) who, while accepting 

 Remak's theory as a whole, introduced certain modifications. 

 Unfortunately, conceiving the sclerotome or Wirbelkerjimasse to 

 be identical with the primitive vertebral centrum of Remak or the 

 intervertebral condensation, Gegenbaur introduced some confu- 

 sion which is scarcely yet cleared away, but he rightly maintained 

 that the apparent spaces which separate successive midsegmental 



