2 COMPARATIVE ANATOMY. 



By an organ we understand an apparatus constructed to perform 

 a definite physiological function, as, for instance, the liver, which 

 secretes bile, the gills and lungs, in which an exchange of gases 

 is effected with the surrounding medium, and the heart, which 

 pumps blood through the body. 



The organ-systems, which will be treated of in order in this 

 book, are as follows : 1. The outer covering of the body, or 

 integument; 2. The skeleton; 3. The muscles, together with 

 electric organs; 4. The nervous system and sense-organs; 

 5. The organs of nutrition, respiration, circulation, 

 excretion, and reproduction. 



The closely-allied branches of science denned above are united 

 together as Morphology, as opposed to Physiology, which 

 concerns the functions of organs, apart from their morphological 

 relations. 



Morphology alone leads us to a satisfactory explanation of the 

 structural phenomena of the animal body, for it not only reveals to 

 us the law of heredity and the consequent relationship of 

 animals to one another, but it also helps to explain certain de- 

 graded and rudimentary forms, which, considered as isolated 

 adult animals, would always remain absolutely incomprehensible. 

 Further, it shows us on the one hand how the animal organism 

 is acted upon by the influence of its surroundings, and how it 

 is apt to change gradually and more or less continuously ; and on 

 the other hand how the capacity of adaptation resulting from 

 these changes varies inversely with the persistence of inherited 

 qualities. These two important opposing factors, adaptation 

 and heredity, constitute the formative principle of the 

 animal body. 



II. DEVELOPMENT AND STRUCTURAL PLAN OF THE 

 VERTEBRATE BODY. 



The structural elements described in the preceding section as 

 the build ing- stones of the organism, i.e. the cells, all arise from a 

 single primitive cell, the egg-cell or ovum. This forms the 

 starting-point for the entire animal-body, and a somewhat full 

 account of its structure and relations must therefore be given here. 

 The stages in its development will only be described in a very 

 general manner. 



The unimpregnated ovum consists of a rounded vesicle (Fig. 1), 

 in the interior of which the following parts can be distinguished : 

 The vitellus (D), the germinal vesicle (KB], and one or more 

 germinal spots (K&). Of these, the germinal vesicle is the most 

 important physiologically (comp. p. 3). The outer-covering of the 

 ovum is spoken of as the vitelline membrane. 



