308 BULLETIN OF THE 



the extent of the group whose origin is sought. The other method is 

 common both to comparative embryology and to comparative anatomy ; 

 it is dependent upon the inherent improbability of the same physio- 

 logical requirements, being met by the same structural device in two 

 groups which are not genetically related ; it can employ equally well, 

 though with a somewhat different significance, both ccenogenetic and 

 palingenetic characters ; it is purely anatomical in its method, and it is 

 in the broadest sense comparative. The first I may designate as the 

 method of elimination, or the intensive method ; the latter as the 

 comparative, or extensive method. 



1 have been led to make the preceding analysis in order to employ the 

 division thus indicated in the subsequent discussion, and also because it 

 is a contrast which does not appear to be generally recognized. Thus, 

 a recent text-book of zoology (Hatschek, '88, pp. 25, 2G) identifies the 

 methods of embryology with those of comparative anatomy, and declares 

 that palingenetic and ccenogenetic characters are equally valuable for 

 phylogenetic inferences. According to the preceding analysis, these two 

 statements are partial, relating only to the comparative method in em- 

 bryology, and ignore the higher use which renders embryological facts 

 of peculiar value. 



Observing then this two-fold division in the following discussion, an 

 attempt will first be made to reconstruct from the ontogeny of Verte- 

 brates the ancestral history of their excretory organs. 



The most general character which appears to be common to the ontogeny 

 of all Vertebrates is the intimate relation which exists between the excre- 

 tory tubules and the coelom. This relation is peculiarby well illustrated 

 by the pronephros, but it is true also of all the urogenital organs, and is 

 a fact which in my opinion throws considerable light on their evolution. 

 The ccelom appears to be an internal cavity developed to meet a num- 

 ber of physiological needs. It is likely that in the lower Invertebrates 

 the ccelom served largely a nutritive function (see, e. g., Chun, '80, pp. 

 248-253) ; but I am of opinion. that in the higher Invertebrates and in 

 Vertebrates the ccelom early became in large measure an excretory 

 space. This function of the ccelom, inferred from its relations with ne- 

 phridia, is in accord with its situation in the body. Evidently the 

 organs which would be most in need of a near place of discharge for 

 nitrogenous waste products are those which are in the highest degree 

 metabolic. Such are, par excellence, the muscle masses of the body, 

 and it is a familiar circumstance that in all Chordates the primitive 

 muscle plates develop from the lining wall of the dorsal segmented por- 



