;',78 Comparative Anatomy — Its History, Aim, and Method 



resemble the form of any skull. But these changes in the form of the 

 model would involve no change in the relative position of the areas 

 circumscribed by the painted lines. With elongation of the model, the 

 individual areas would become longer and narrower, but whatever the 

 change of form, there would be no change in the position of any one 

 area in relation to contiguous areas. So the study of the common pat- 

 tern in various skulls reveals very little "slipping" of bones out of their 

 usual relations to neighboring bones. It may occur, but rarely goes far 

 enough to obscure the identity of a bone. The main causes of confusion 

 for the anatomist lie in the occasional adding, omitting, or fusion 

 of bones. 



There appears to be no reason for doubting that the uniformity in 

 the general plan of structure of vertebrate skulls depends on inherit- 

 ance. Were it not so — that is, if the embryo were free to produce its 

 own peculiar type of skull in the quickest and easiest way — it surely 

 would not employ the indirect and laborious methods which are espe- 

 cially characteristic of the development of skull bones. Beyond ques- 

 tion, in the building of the skull, the embryo, bound by inheritance, 

 begins by using the old material and the old methods. The definitive 

 adult structure is achieved in later stages, but at the cost of much re- 

 forming and even replacement of parts first constructed. A primary 

 skull is built up of cartilage, but most of it is destined to be destroyed 

 and replaced by bone. Granting that the general plan of the skull is 

 inherited, it becomes the task of the anatomist to ascertain the homol- 

 ogies of the bones. The highly complex and variable form of the skull 

 as a whole, the great number of bones which enter into it, the intricate 

 forms and connections of individual bones, and the obscurity in the 

 embryonic origin of many of them, combine to make the homologies 

 of these bones a most difficult field of study. In undertaking it, the 

 anatomist must rely mainly on relative position, the "principle of 

 connections," and embryonic origin. 



There are some cases in which an organ seems to have "dragged 

 anchor." In the common cod and similar fishes (hake, haddock and 

 pollock), there are two pairs of fins (Fig. 299). Those of one pair are 

 larger, situated well up on the side of the body and close behind the 

 gills. Those of the other pair are smaller, situated near the midventral 

 line and anterior to those of the other pair. The distance between the 

 two pairs varies in the several species. By definition and in conformity 

 with the "general body-plan" of a vertebrate, pectoral appendages are 

 anterior and pelvic are posterior. Primitively, the pelvic fins of fishes 

 are near the anus and have no skeletal connection with the vertebral 

 column. The pectoral fins are more strongly developed, have a more 

 elaborate internal skeleton, and commonly have strong skeletal attach- 



