290 J- B. Haycraft, 



for purposes of description, the primitive renal vesicles. In Fig. 4 

 the letter A is placed in this first vesicle, and it can be noted that 

 its epithelium is prolonged into two solid prolongations of cells which 

 have evidently proliferated from it. Although the basement mem- 

 brane is not continued around these prolongations — the commencing 

 tubules — their epithelium, from the "set" and character of the cells, 

 is clearly separated from the surrounding blastema, which is diffe- 

 rentiating into connective tissue. Both the cells of the blastema and 

 the epithelial cells of the primitive renal vesicle are rapidly prolifera- 

 ting as indicated by the frequent occurrence of Karyokinetic figures. 



Embryo rabbit of twelve days. 



In the embryo rabbit of twelve days the ureter has passed up 

 the middle of the Kidney blastema on its mesial side, has divided into 

 two or three branches these again have divided 

 to form six tubules all of which are terminated 

 -F by primitive renal vesicles. These are spherical 

 cavities compressed in a direction vertical to the 

 cortex and two or three times the diameter of a 

 tubule. In this embryo, in which the Kidney mea- 

 sures 0,025 cm across, the diameter of the tubule 

 = 0,0025 cm while that of the vesicles = 0,0075 cm. 



Diagrammatic view At this Stage the vesicles are lined by columnar 

 of the Kidney of a 

 12 days rabbit. cells placed upon a distmct basement membrane. 



It may be well at this point to illustrate the 

 method by means of which the above results were arrived at, a 

 method adopted throughout the whole of this investigation. The 

 sections mounted in series were examined one by one commencing 

 at the tail, and when say the right Kidney first appeared in the 

 section, a drawing of the section was made. The other sections were 

 subsequently di^awn in series Fig. 6 and in the illustration nine draw- 

 ings of the lower half of the right Kidney were made, from which 

 part of the diagram Fig. 5 was constructed. In a the ureter and 

 part of a primary renal vesicle are cut across, at a higher plane (b) 

 the vesicle is seen cut through its centre, at (c) a second vesicle is 



