MUSEUM OF COMPARATIVE ZOOLOGY. 301 



tion actually realizes a given character, or merely shows a tendency to 

 assume it, provided in the latter case it can be satisfactorily shown that 

 the realization of the tendency was prevented by intelligible causes. 

 Thus, in the gastrulation of meroblastic eggs, if it be recognized that 

 the great accumulation of yolk renders eniboly impossible, the substi- 

 tution of epiboly in these cases must be regarded as morphologically 

 insignificant. 



The question now naturally arises, Are any of the contrasts between 

 pronephros and mesonephros of such a nature that they can be explained 

 as the result of a single modifying influence ? As I have already stated, 

 the most marked point of disagreement between the two glands is the 

 difference in time at which they appear. What influences may that 

 factor exert in modifying their development] At the time when the 

 Amphibian mesonephros appears, the myotomes are widely separated 

 from the peritoneum, and the continuous strip of ccelom immediately 

 ventral to the lower boundaries of the protovertebrse in the region of the 

 pronephros does not exist in the region of the body in which the meso- 

 nephros develops. In its place is a mass of cells which extends from the 

 dorsal angle of the body cavity upward towards the overlying myotomes. 

 This mass of cells has been regarded as the first rudiment of the meso- 

 nephros. The most natural explanation of the condition is that this mass 

 of cells is morphologically not a secondary proliferation from the perito- 

 neum, but is really the last remnant of the mesoderm which formerly 

 connected the dorsal angle of the permanent body cavity with the over- 

 lying protovertebrse. The correctness of this interpretation is shown by 

 comparison with the conditions in Selachians and in Amniotes, where, 

 according to the mutually confirmatory accounts of Sedgwick ('80 a ), Van 

 Wijhe ('8S a , '89), Ruckert ('88), and Hoffmann ('89), the mesonephric 

 tubules develop from the communicating canal. The first rudiment of 

 each mesonephric tubule is in reality that portion of the primitive meso- 

 dermal plate which lies immediately ventral to the protovertebrse, and, 

 corresponds to that portion of the ccelom into which, as shown in Figure 6, 

 the glomus projects, and from which the pronephric tubules emerge. 

 Each mesonephric fundament, then, presents on its outer side somatic, on its 

 inner, splanchnic mesoderm. When the fundaments of the mesonephros 

 have been converted into a series of blind tubules, they grow outward and 

 join the segmental duct. This process appears to me to be precisely 

 equivalent to the somatopleural evagination, which at an early period gave 

 rise in the anterior region to the nephrostomal tubules of the pronephros. 

 That portion of the differentiated mesonephric tubule into which the 



