4 T. H. BURLEND, 



tubules by estimation, or bj' identification with their earlier 

 stages. 



This investigation has been carried on partlj^ at University 

 College, Cardiif, where I have received much encouragement and 

 many facilities from Prof. W. N, Parker; also in the Zoological 

 Laboratory at the Royal College of Science, where the late Prof. 

 A. Sedqwick was ever ready to help me with friendly criticism and 

 advice: to both of whom 1 wish to record my gratitude. I have to 

 thank Mr. M. Y. Orr and Mr. H. A. Haig M. B. of University College 

 Cardiff; for their cooperation in the taking of the micro-photographs 

 which appear in the Plates at the end of this paper. 



Details of observations. 



Embryos A — H show the early condition of the kidney rudiment, 

 up to the stage when it begins to differentiate into duct (Sammel- 

 gang) and pronephric tubules. 



Embryos I— N show embryonic stages where the pronephros is 

 attaining or has attained to its maximum development, and where 

 the anterior part of the mesonephros has become evident. 



Embryos — S exhibit the pronephros at various stages of de- 

 generation, the mesonephros being predominant. 



Embryo A. 



This embryo was stained in Boraxcarmine for 45 hours, sectioned, 

 and mounted on 5 slides with 38, 39, 40, 38 and 38 sections upon 

 them respectively. 



I could distinguish 6 proto vertebrae in the embryo after clearing 

 in xylol preparatory to sectioning. The following table will be useful 

 in facilitating the description of the kidney. (T he n u m b e r i n f r o n t 

 of the comma indicates the number of the slide, the 

 number after the comma indicates the section on the slide.) 



Body begins 1,1. 



Left side of embryo. 



Mesoderm begins 1,18; no signs 



of segmentation behind the ear 



until 2,7 



1,26—1,36 Ear 1,26—1,35 



2,1?— 2,7 (rudimentary) I 1,38-2,5? 



2,7 —2,14 II ■ 2,5?-2,12 



Right side of embryo. 



Mesoderm begins 1,19; no distinct 



trace of segmentation behind tiie 



ear until 2,5 



