6 EMBRYOLOGY. 



Numerous examples of the same phenomenon are found amongst 

 the Crustacea. 



(4) How far organs appear in the embryo or larva which 

 either atrophy or become functionless in the adult state, and 

 which persist permanently in members of some other group or 

 in lower members of the same group. Cases of this kind are of 

 the most constant occurrence, and it is only necessary to cite 

 such examples as the gill-slits and Wolffian body in the embryos 

 of higher Craniata to illustrate the kind of instance alluded to. 

 The same conclusions may be drawn from them as from the 

 cases under the previous heading. 



(5) How far organs pass in the course of their development 

 through a condition permanent in some lower form. Phylo- 

 genetic conclusions may be drawn from instances of this cha- 

 racter, though they have a more important bearing on Organology 

 than on Phylogeny. 



The considerations which were used to shew that the an- 

 cestral history is reproduced in the ontogeny of the individual 

 apply with equal force to the evolution of organs. The special 

 questions in Organology, on which Comparative Embryology 

 throws light, may be classified under the following heads. 



(1) The origin and homologies of what are known as the 

 germinal layers; or the layers into which the embryo becomes 

 divided immediately after the segmentation. 



(2) The origin of primary tissues, epithelial, nervous, mus- 

 cular, connective, etc., and their relation to the germinal layers. 



(3) The origin of organs. The origin of the primitive 

 organs is intimately connected with that of the germinal layers. 

 The first differentiation of the segmented ovum results in the 

 cells of the embryo becoming arranged as two layers, an outer 

 one known as the epiblast and an inner one as the hypoblast. 

 The outer of these forms a primitive sensory organ, and the 

 inner a primitive digestive organ. 



(4) The gradual evolution of the more complicated organs 

 and systems of organs. 



This part of the subject, even more than that dealing with 

 questions of Phylogeny, is intimately bound up with Com- 

 parative Anatomy; without which indeed it becomes quite 

 meaningless. 



