THE SKELETON OF THE VERTEBRATE HEAD. 97 



tube can be considered as a proper haemal arch, its development 

 must have been ascertained, or its relations to those muscular, 

 vascular, but more particularly nervous elements, which 

 constitute in their respective systems the arrangements cor- 

 responding to the haemal arch in the sclerotome, must have 

 been determined. 



I must confess therefore my inability to discover the precise 

 view of the haemal arch taken by Professor Owen. Judging 

 from his diagram of the " ideal typical vertebra," and from his 

 general treatment of the subject, a chevron bone in the reptile 

 or mammal, or that portion of the cervical vertebra in certain 

 birds which completes the canal beneath the centrum, repre- 

 sents the primary typical form of this arch. It would also 

 appear to follow from his doctrine that the expanded form of 

 haemal arch, provided for the lodgment of the central organ of 

 circulation, and presented by the thoracic segments, is a 

 secondary formation the result of the removal of the primary 

 haemal arch from its "typical" position under the centrum, 

 and its intercalation between the elongated pleurapophyses. 

 But this doctrine appears to me to involve embryological con- 

 tradictions. The relations of these primary and secondary 

 forms of haemal arch in the neck and throat respectively are 

 not explained by it. The so-called haemal arch under the 

 cervical vertebra of the pelican is undoubtedly haemal in 

 function ; but as it excludes the oesophagus and trachea, it 

 cannot be the real or morphological haemal arch. In other 

 words, this so-called haemal arch cannot have been formed in 

 the " ventral folds " of the embryo neck. 



Again, it is difficult to conceive how the pleurapophyses 

 and haemal arch of Professor Owen's " ideal typical vertebra " 

 can be developed together in the " ventral folds " of the embryo. 

 For, according to the doctrine of Professor Owen, a pleurapo- 

 physis may, in different instances, present two sets of relations. 

 In the thorax it is attached by opposite ends to adjoining 



