INTRODUCTION. 5 



(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. 



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

 through a condition permanent in some lower form. Phylogenetic 

 conclusions may be drawn from instances of this character, though 

 they have a more important bearing on Organology than on Phylogeiiy. 



The considerations which were used to show that the ancestral 

 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, maybe 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 im- 

 mediately after the segmentation. 



(2) The origin of primary tissues, epithelial, nervous, muscular, 

 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 Comparative 

 Anatomy ; without which indeed it becomes quite meaningless. 



REPRODUCTION. 



A study of reproduction logically precedes that of Embryology. 

 Reproduction essentially consists in the separation of a portion of an 

 organism which has the capacity of developing into a form similar 

 to that which gave it origin. The simplest modes of reproduction 

 are those which occur amongst the Protozoa. 



In this group, reproduction may take place in a great variety 

 of ways. These may be classified in three groups : (1) fission, 

 (2) budding or gemmation, (3) spore formation. 



Reproduction in all these ways may take place either subsequently 



