302 BULLETIN OF THE 



glomerulus projects is of different origin ; it is merely a portion of the 

 coelom, the walls of which are to be understood to be formed as I have 

 just stated in part by somatic, in part by splanchnic mesoderm. 



Returning now to the two features in which the glomus was shown 

 to differ from the glomeruli, — viz. situation within the body cavity, and 

 continuity throughout successive somites, — it will be seen that it is im- 

 possible to maintain the former as a ground of distinction, since the 

 glomerulus also lies in a detached portion of the coelom, and that the 

 latter ground is equally untenable because it simply results from the fact 

 that, before the glomeruli appear, the space iuto which they would other- 

 wise project as a continuous organ has already been broken up into a 

 series of distinct tubes ; the glomerular organ is consequently broken 

 up iuto a corresponding number of separate vascular processes, each of 

 which becomes converted into a Malpighian capsule. 



It seems probable, therefore, (1) that the pronephros and mesonephros 

 were primitively alike, and were portions of a single continuous gland ; 

 (2) that in Vertebrates which came to lead an independent existence 

 early in life, an anterior portion of the gland and the whole of the duct 

 are differentiated before the posterior part for the immediate purposes of 

 the larva; and (3) that the difference in structure between the two 

 glands is mainly due to their arising at different times relatively to the 

 differentiation of the body cavity and protovertebrse. Applying this 

 conclusion to the tubular portion of the glands, it becomes at once intel- 

 ligible why the tubules of the mesonephros must of necessity join the 

 duct secondarily. From this standpoint, the existence of convolutions 

 in the common trunk points to a less differentiated condition of the 

 pronephros, in that, for temporary purposes, the longitudinal canal, 

 including the common trunk, subserves at the same time the functions 

 of an efferent duct and of a secreting tubule. 



The foregoing explanation of the nature of the pronephros is based 

 upon the assumption that it is developed as a larval excretory organ. 

 In order to justify this position, it will be necessary to consider whether 

 the pronephros is functional in those Vertebrates which, viewed from this 

 standpoint, would seem to require this organ, and in such alone. For 

 the present purpose, two methods of sexual reproduction may be dis- 

 tinguished : (1) that in which the mother spends her energy in pnxhicing 

 a large number of offspring, which are early forced to care for themselves ; 

 and (2) that in which the mother produces a small number of eggs, and, 

 either by giving to each a large quantity of reserve food yolk, or by 

 nourishing the young embryo within her own body, secures the existence 



