66 HISTORY OF THE HUMAN BODY 



definite ccelom, or pleuro-peritoneal cavity, of which they fur- 

 nish the lining membrane, the peritoneum. The outer layer 

 (parietal mesoderm) lines the body wall; the inner (visceral 

 mesoderm) invests the primary intestine and, later on, its 

 derivative organs, as lungs, pancreas and liver. In all ex- 

 cept mammals the membrane is a continuous one, but here, 

 through the formation of the diaphragm and the consequent 

 setting apart of a separate thoracic cavity, the portion thus 

 cut off is treated as a distinct membrane and called the pleura. 



Although the above sketch represents the underlying plan 

 upon which the development of all vertebrates is based, it is 

 not found in an unmodified condition save in the lowest classes. 

 It is most typically represented in the development of Am- 

 phioxus, for which the foregoing description, save in a few 

 points, might well be used; in the selachians, also, the modi- 

 fications are not very great and the plan may be easily traced. 

 In the amphibians, however, the plan is so much obscured, 

 especially in its earlier stages, that for a long time, during the 

 early history of the science of embryology, the homologies 

 were not recognized. These modifications become still greater 

 in the Sauropsida and Mammalia, in which, without the help 

 of the amphibians, which here, as elsewhere, form a valuable 

 connecting link, the recognition of the early stages would be 

 hardly possible. The principal disturbing factor, at least in 

 amphibians and the sauropsida, is the presence of increasingly 

 greater quantities of yolk, which presents numerous mechan- 

 ical problems, and its influence is felt with equal emphasis in 

 the case of placental Mammals, where the egg, although yolk- 

 less, has evidently become so through a secondary reduction 

 and still follows in its development that of the yolk-filled eggs 

 of the Sauropsidan type. 



One of the most important modifications in the develop- 

 mental history of the higher classes concerns the appearance 

 and subsequent development of the mesoderm and the forma- 

 tion of the definite ccelom. In Amphioxus the pairs of di- 

 verticula arise in quite typical fashion from the sides of the 

 primitive intestine, and this procedure is almost as easily 



