98 ON THE MORPHOLOGICAL CONSTITUTION OF 



sclerous elements, and lies in the wall of the hfemal chamber. 

 In the neck and tail it is connected to its own vertebra at one 

 end only, and does not lie in the wall of the htemal chamber. 

 The mode in which the continuously-arranged elements of the 

 costal arch of a bird — 'the " pleurapophyses," " hremapophyses," 

 and " h!3emal spine " — are developed in the embryo is known. 

 But it is difficult to conceive how the detached and peculiarly- 

 arranged " pleurapophyses " and " haemal arch," as represented 

 in the " ideal typical vertebra," or exemplified in a proximal 

 caudal vertebra of a reptile or perenni-branchiate amphibian, 

 have assumed the positions they occupy, if they belong to the 

 same group of elements — that is, if they all spring from or 

 originate in the wall of the visceral chamber. 



Is the pleurapophysis a fvmdamental or primary element 

 of the hsemal arch ? In other words, is it originally developed 

 in the wall of the visceral cavity, and in certain instances 

 afterwards extruded from it ? or is it merely a secondary ele- 

 ment in the hsemal arch — that is, formed externally to, or away 

 from it, and only intercalated into it in certain vertebrae ? 



As a rib, so far as its development has been traced in the 

 series, appears to be formed in the inner layer of the " ventral 

 fold ;" and as it is previously connected or continuous with 

 the diapophyseal portion of the neurapophyses, its head and neck 

 being secondary formations, I am inclined to consider the caudal 

 transverse processes in the mammal, lizard, and amphibian as 

 lying in the position of the original " ventral folds," and that, 

 therefore, the feebly-developed " pleurapophyses " of this region 

 are the only representatives of its haemal arches, while the 

 chevron-bones have no title to this morphological distinction.* 



* In dissecting lately a large crocodile, I found that an aponeurotic 

 membrane extended outwards and curved dowTiwards on each side from the 

 extremities of the caudal transverse processes. These aponeuroses met one 

 another in the mesial line below the tail, and were there joined by a mesial 

 aponeurosis which extended down from between the chevron-bones. A layer 

 of fat one-third of an inch in thickness lay on the outside of the lateral apo- 



