MUSEUM OF COMPARATIVE ZOOLOGY. 299 



ovarj than in the testis, even though the latter organ is in such intimate 

 rekxtions with degenerating Wolffian canals. 



The remaining portion of the present paper will be concerned with 

 those inferences of a general nature which can be drawn from the develop- 

 ment of the pronephros and segmental duct as traced in the preceding 

 pages. These general conclusions naturally fall into two groups : (1 ) such 

 as are of principal interest in elucidating the development of the excre- 

 tory system, and (2) such as tend to throw light on the development of 

 the Vertebrate type. Following this division, then, in our discussion, I 

 shall consider in the present section the organogenetic conclusions ; and, 

 in concluding, deal with the phylogeuetic conclusions which seem to me 

 waiTranted by our present knowledge. 



In the historical review of the development of the pronephros, it proved 

 in several groups very difficult to draw a sharp line between the pro- 

 nephros and the mesonephros, and it was suggested at that point in the 

 discussion that this difficulty is in reality a fundamental one, and one 

 which is indicative of the true relations between these parts. The 

 question of the serial homology of the pronephros and mesonephros, as 

 it presents itself to the modern student, is to my mind simply this : 

 Are we to regard these two glands as derivatives of one continuous 

 ancestral organ, which at one time extended over all the somites now 

 occupied by each 1 The answer to this question naturally must come, if 

 at all, by a comparison of the two organs for the purpose of bringing to 

 light their features of similarity and those of contrast. Manifestly they 

 differ in the time of their appearance ; indeed, from this circumstance 

 the two glands were distinguisked and named ; it remains to consider 

 whether they are constructed on the same or on different types. 



In endeavoring to furnish an answer to this question, I shall proceed 

 to an anatomical comparison of the glands, taking into consideration both 

 of the principal portions involved, the glomerular and the tubular parts. 

 The glomerulus of the mesonephros resembles the glomus of the pro- 

 nephros in the following particulars : both are highly vascular structures 

 composed of ramifying blood-vessels and mesenchyme ; they project into 

 spaces which are in communication with the exterior by means of excre- 

 tory conduits; they originate outside of this space, and gain position 

 within it by pushing before them in their growth its epithelial wall, which 

 then persists as an outer covering to the vascular process ; they receive 

 branches directly from the aorta ; and, finally, they are developed in re- 

 gions of the body which at least nearly correspond to each other serially, 



