500 ' 



pronephric tubules arise in segments lY to X (ranging from 7 to 14 

 in number); that mesonephric tubules extend as far forwards as seg- 

 ment VI; that in later embryos pro- and mesonephric tubules are 

 indistinguishable; that the mesonephric funnels situated in the 

 pronephric region open into the coelom. 



Many other authors have dealt with the development in other 

 animals of the structures which I have examined, and I cannot claim 

 originality on any point below mentioned since the subject has been 

 worked at so extensively: I am inclined to suggest that the mode of 

 transition from glomus to glomeruli and from pronephric tubule to 

 mesonephric has not hitherto been regarded in the same light by 

 previous observers. A comprehensive comparison of the position taken 

 up by the leading authorities, and the necessary change of aspect 

 which my results, if substantiated, would involve, must be deferred 

 until a detailed account of my observations have been given. 



I hope to publish such an account, with numerous illustrations 

 in the near future. 



Material and observations. 



The material first used was Cheloue viridis, including one embryo 

 on the blastoderm (8 m.m. long) and many later stages from 10 m.m. 

 upwards. Owing to the characteristic flexure of the head in these 

 Reptiles, and the disproportionately small growth lengthwise Avith 

 increasing age, it is better to compute the age of the early embryo 

 by counting the number of protovertebrae. These embryos were well 

 preserved and stained well in bulk with Ehrlich's Haematoxylin. 

 Transverse and longitudinal-vertical sections were taken in order to 

 follow the course of the individual tubules at the front end of the 

 kidney. A camera lucida drawing' was then made of each succes- 

 sive section and correctly numbered : from these drawings it was 

 possible to mark out, upon squared paper, the course of each tubule, 

 and thereby to construct a rough model of the relative position of the 

 tubules. This somewhat arduous piece of work was necessary in order 

 to determine the complex interwindings of the tubules and their 

 occasional branching. My attempts at making reconstruction models 

 of the anterior end of the kidney in the ordinary way were not, 

 owing to the nature of the kidney, successful. 



I have recently procured some well-preserved embryos of an 

 American Terrapin — Chrysemys marginata — the younger ones fixed 



