PRINCIPAL MODIFICATIONS OF THE VERTEBRATE TYPE. . 7 



very bulky mesoderm, which is divided by large cavities (abdominal and thoracic) 

 into two main layers, one of which is closely associated with the epidermis and 

 forms the body wall, the somatopleure of embryologists; the other joins with the 

 entoderm to complete the walls of the splanchnic viscera, and constitutes the 

 splanchnopleure of embryologists. The mesoderm is permeated by two sets of 

 cavities: (i) the heart and blood-vessels; (2) the lymphatic system. It is also 

 differentiated into numerous tissues (muscles, tendon, bone, etc.) , organs and the 

 internal parts of the urogenital system. The nervous system, although developed 

 from the ectoderm, is found separated from its site of origin, and completely en- 

 cased in mesoderm. 



The Principal Modifications of the Vertebrate Type. 



Our knowledge of human development being at the present time incom- 

 plete, it is often necessary to supplement that knowledge by reference to facts 

 of observation on the development of various vertebrates. Indeed, the best study 

 of human embryology includes more or less comparative work. We shall, therefore, 

 find frequent occasion to refer to the development of many vertebrate types. 

 Accordingly, in this section there are given definitions of the principal subdivisions 

 of the vertebrates to which we shall have occasion to refer. 



From an embryological standpoint, vertebrates may be separated into two 

 main divisions, which are commonly designated as the Amniota and Anamniota, 

 distinguished by the presence or absence of the amnion, the amnion being a thin 

 membrane, which immediately surrounds the embryo in the higher forms. It 

 occurs in reptiles, birds, and mammals, which together constitute the Amniota. 

 It is absent in the fishes and amphibians, which therefore constitute the Anamniota. 

 These two divisions are also distinguished by other peculiarities. The higher forms 

 referred to all have the organ known as the allantois, an appendage of the embryo, 

 which is lacking in the lower forms. The comparative anatomist finds many 

 points of resemblance between the various classes of fishes, on the one hand, and 

 the amphibia, on the other, and indicates this relationship by the use of the term 

 Ichthyopsida, which means "fish-like." In our present classification the term 

 Ichthyopsida is synonymous with Anamniota. The comparative anatomist further 

 recognizes a close relationship between birds and reptiles, and puts these together 

 under the common designation of Sauropsida, or "reptile-like." 



As regards the fishes, many classifications are more or less in vogue at the 

 present time. For the purposes of this book, the following names for the classes have 

 been adopted as names generally understood and sufficiently exact to meet our 

 needs: The lowest fishes are the hag-fishes and lampreys, constituting the group 

 of Marsipobranchs. Next comes the group comprising the sturgeon and its allies, 

 for which we have retained the old term of Ganoids. To these fishes the central 

 position in the system must be assigned, and it is probable that the higher fishes 

 are more or less directly descended from Ganoid-like forms. They fall into three 



