74 T. H. BURLEND. 



b) Whether the two processes develop at an equal or a diffe- 

 rent rate, 



AMth regard to (a) I will deal with the case of Chrysemijs only. 

 Here the kidney growth (if we judge by the duct) slightly precedes 

 the process of metamerism. Hence we should expect to (and do) 

 find that the pronephros, at least its anterior portion, is non-meta- 

 meric. The conditions may not be the same for other animals. 



With regard to (b) a reference to Tables A — J shows that 

 the rate of advance of the metamerism is greater than the rate 

 of advance of kidney growth: hence the posterior part of the body 

 becomes segmented before the kidnej' has extended back to the cloaca. 



Thus we should expect to find some parts of the kidney 

 (postr region of pronephros perhaps) affected by the metamerism, and 

 also the whole length of the mesonephros. Add to this the fact that 

 the nephrotome (which is metameric in most of the trunk segments 

 of the body) takes part in the formation of the mesonephric tubules, 

 and we account for the early condition of the mesonephros in the 

 Dogfish. 



But the metamerism is not a permanent condition, since at an 

 early stage in development it begins to lose ground (more particularly in 

 the posterior region of the body). Hence the non -metameric condition 

 of the posterior end of the mesonephros and of the metanephros in 

 most animals. 



What I have said about the influence of environmental factors 

 upon the tubules themselves applies also to the blood-supply, and 

 thus we may account for the condition of the glomus or the glomeruli 

 in the kidney in different Vertebrates. This matter I must however 

 defer until I deal with the mesonephros in Chrysemijs. Two other 

 questions which must be left at the present stage are: 



a) The later changes in structure of the pronephric nephrostomes;. 

 Subdivision probably occurs. Felix says that in the Amniote 

 pronephros no further differentiation of the tubules occurs, but the 

 evidence of Chrysemys does not bear this out. See also Maas (1897), 

 Price Myxine (Stage C). 



b) The changes occurring in the lower end of the nephrotomes. 

 Does this region become swollen in each segment so that the 

 successive spaces form an overlapping series? The evidence of 

 Lepidosteiis , IchthyopJiis, Chrysemys etc. renders this probable, and 

 so leads up to the condition in the posterior mesonephros and 

 in the metanephros. 



