600 MONTGOMERY— MORPHOLOGY OF THE [Apul2+, 



latter, i. e., as a longitudinal peritoneal groove, showing sometimes 

 (Reptiles) traces of segmental origin, that becomes a tube closed 

 from the coelom except at its anterior end (ostium). These two 

 kinds of Miillerian ducts cannot be homologized, for the first is an 

 abstriction from the segmental duct, while the second arises as a 

 peritoneal infolding and may be compared with an elongated peri- 

 toneal funnel or with a series of them. The ovaries differ from 

 testes in lacking vasa efferentia connecting them with the ducts, but 

 other remnants of mesonephric tubules are found in amniotes in 

 form of the epoophoron and paroophoron. 



Other Excretory Organs. — The liver forms urea, while the 

 sudoriparous glands, respiratory organs and skin aid in the dis- 

 charge of waste substances. 



B. GENERAL COMPARISONS. 

 I. Main Types of Excretory Organs. 



We use the idea homology to denote that relation between a 

 certain organ of one animal and a certain organ of another, which 

 is dependent upon derivation from a common ancestral organ. In 

 other words, homology denotes community of descent of parts. 

 To elucidate such relations, to demonstrate change of both form 

 and use of parts, is the first object of comparative anatomy; later all 

 such knowledge may be so compounded as to give the general his- 

 tory of phylogeny. When one considers such manifold and diverse 

 organs as those that subserve excretion, difficulties of interpretation 

 that are almost insuperable arise to perplex and bewilder, yet at 

 the same time compel, the attention. Any conclusions with regard 

 to the homologies of these organs must be tentative because our 

 knowledge of them is so very imperfect; in fact for most of the 

 animal groups only the outlines have been made known. Therefore 

 the following attempt to arrange the excretory organs according 

 to their genetic relations should be regarded as only an essay. 



The criteria of homology are still a matter of dispute. I have 

 discussed this matter in another place (1906), and will simply state 

 here that similarity of relative position to other parts seems to be 

 the surest criterion, together with general similarity in mode of 



