154 THE FOURTH DAY. [CHAP. 



immediately preceding. Similarly, the rudiment of each 

 vertebral arch covering in the neural tube no longer springs 

 from the hind part of the protovertebra from which it is an 

 outgrowth, but forms the front part of the permanent ver- 

 tebra, to which it henceforward belongs. The ganglia are still, 

 however, the most conspicuous portions of each segment. 



By these changes this remarkable result is brought about, 

 that each permanent vertebra is formed out of portions of 

 two consecutive proto vertebrae. Thus, for instance, the tenth 

 permanent vertebra is formed out of the hind portion of the 

 tenth protovertebra, and the front portion of the eleventh 

 protovertebra, while its arch, now attached to its front 

 part, was attached to the hind part of the tenth protovertebra. 



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 proto vertebral bodies surrounds and invests the 

 notochord, has not only increased in mass but also has 

 become cartilaginous, so that, as Gegenbaur (Untersuchung 

 zur vergleichenden Anatomic der Wirbelsdule bei Amphibien 

 und Reptilien, Leipzig, 18G2) points out, we have for a short 

 period on the fifth day a continuous and unsegmented carti- 

 laginous investment of the notochord. 



This cartilaginous tube does not however long remain uni- 

 form. At a series of points corresponding in number to the 

 original proto vertebrae it becomes connected with a number 

 of cartilaginous arches which appear in the protovertebral 

 investment of the neural canal. These arches, which thus 

 roof in the neural canal and each of which arises opposite to 

 the vertebral portion of each protovertebral body, are the 

 cartilaginous precursors of the osseous vertebral arches. We 

 further find that the portions of the cartilaginous tube from 

 which the arches spring come to differ histologically from the 

 portions between them not connected with arches : they are 

 clearer and their cells are less closely packed. There is 

 however at this period no distinct segmentation of the 

 cartilaginous tube, but merely a want of uniformity in its 

 composition. 



The clearer portions, from which the arches spring, form 

 the bodies of the vertebrce, the segments between them the 

 intervertebral regions of the column. 



