12 . T. H. BüRLEND, 



ment IX, with a duct extending as far back as the end of seg- 

 ment X. 



On the right side the conditions are similar: the outgrowth 

 becomes clearly visible at 3,35 and extends continuously backwards 

 as far as 4,31, after which the duct is separate and ends at 5,6. The 

 few cells at its hind end appear as though split off from the meso- 

 derm in this region. At 4,29 there is visible for the first time 

 the characteristic flexure which the outgrowth undergoes at a later 

 period. 



The region affected on the right side extends from about the 

 posterior end of segment V, VI, VII, VIII and the beginning of 

 segment IX, behind which the duct extends to the first 2 or 3 sections 

 of segment X. 



The rudiment of the kidney — the "primitive kidney groove" — 

 is more in the nature of a solid ridge of cells rather than a groove, 

 except where the cavity of a protovertebra is in communication 

 with the general coelom. 



The history of segment VIII, for example, is instructive since 

 it shows that the kidney rudiment first appears as a primitive 

 kidney groove in the somatic mesoderm of the lateral plate region ; 

 a more or less solid outgrowth results from this groove by approxi- 

 mation of its walls; this outgrowth gives rise distal ly to the 

 pronephric duct joining together the anterior pronephric tubules, 

 and proximally, by atrophy of intervening structures to the tubules 

 themselves. In segment IX the duct appears first, then the fusion 

 with it of the mesoderm beneath, and afterwards differentiation into 

 tubules and duct occurs. 



Embryo D. 



Stained for 45 hours in Boraxcarmine, sectioned, and mounted 

 on six slides upon which were 30, 30, 45, 45, 45, and 45 sections 

 respectively. 



Body begins 1,3. 



