22 GENERAL CONCEPTIONS. 



rior lobe is an outgrowth from the cavity of the mouth of the embryo. Com- 

 paratively early in the development of the individual this outgrowth becomes 

 entirely separated from the mouth-cavity (from the walls of which it arose), and 

 forms a closed vesicle. It exists in every known vertebrate animal, has been 

 much studied, but still remains an organ the significance of which we cannot ex- 

 plain. Its absolute persistency and the uniformity of its development indicate 

 that it is an organ of importance, but beyond that we can hardly go. 



To these conceptions, the student should add the following comprehensive 

 morphological notions : The mammalian body may be defined as two tubes of 

 epithelium, one inside the other; the outer tube (epidermal or ectodermal) is 

 very irregular in its form; the inner tube (entodermal) is much smaller in diam- 

 eter, but much longer than the outer and has a number of branches (lung, pan- 

 creas, etc.), and is placed within the ectodermal tube. Between these two tubes 

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

 thoracic) into two main layers, one of which is closely associated with the epi- 

 dermis and forms the body- wall, the somatopleure of embryologists ; the other 

 joins with the entoderm to complete the walls of the splanchnic viscera, and con- 

 stitutes 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, muscle, tendon, bone, etc., and 

 organs, urogenital system. The nervous system, although developed from the 

 ectoderm, is found separated from its site of origin, and completely encased in 

 mesoderm. 



The Principal Modifications of the Vertebrate Type. 



Our knowledge of human development being at the present time very 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 distin- 



