1883.] 233 [Jeffries. 



SUMMARY. 



We now come to the last part of the subject, — to the com- 

 parison of the various structures already described. Before going 

 farther it is as well to explain the meaning of certain words as 

 here used. Animals may be regarded as built on certain types, 

 the animals formed on any type forming groups, genera, species 

 and so on, according to the degree of specialization of the type ; 

 a type being not an ideal form, but the form transmitted by 

 inheritance. It is of course evident that specialization of sub- 

 types may involve the loss of parts as well as their addition. 

 Now, each part of the type form which is separable from the rest, 

 may be regarded as an element. Parts of animals formed from 

 the same elements of the type, may be regarded as morphologous, 

 provided the parts are inherited from the type form. The type 

 forms may have parts like in themselves but differing in their 

 relations to the whole. Such parts may be termed homologous. 

 Finally, parts alike, in a greater or less degree, as to structure, 

 form, position or function, may be termed analogous in so far as 

 they resemble each other. Here it must be borne in mind that 

 completely analogous organs, which do not exist in a major type, 

 may exist in sub-types ; such organs, though having the appear- 

 ance of being morphologous, lack the necessary genetic con- 

 nection. 



Before passing to the appendages it is necessary to review the 

 epiderm, and form a general idea of its nature, to decide what 

 layers are due to transmission, what to simple laws of physiology. 

 The lower, or mucous layer, may be regarded as the epiderm, 

 since from it all other layers are derived. This is of course the 

 most important layer. Next in extent and time of development 

 comes the epitrichial layer. This forms the external coat of all 

 vertebrate embryos and is renewed in cases where a general moult 

 takes place; as among snakes. In embryos it forms from the one 

 layered epiblast in the first stages of growth, or both mucous and 

 epitrichial layers are formed together. Balfour considered the 

 first as the primitive method, and with this opinion we must 

 agree. Accordingly the epitrichial layer is to be regarded as a 

 layer transmitted from some of the early ancestors of the verte- 

 brates, and second only to the mucous layer. 



