No. 3-] THE PRONEPHRIC DUCT IN SELACHIANS. 12J 



FIG. 6. 



another small diverticulum of the aorta. The duct lies close 

 against the mesoderm as far as the 26th somite, where it bends 

 outwards and fuses with the ectoderm opposite the 2/th somite. 

 The fusion continues to the end of the duct opposite the 2Qth 



somite. The pronephros on the left side 

 of the same embryo also begins at the /th 

 somite, but has only five tubules with three 

 aortic diverticula. The end of the left duct 

 may be seen in the figure. The aortic 

 branches on- the right side seem to corre- 

 spond very closely with the descriptions 

 already given by Paul Mayer, Riickert, van 

 Wyhe, and Rabl. On the left there seem to be only wide 

 pockets pushed out from the aorta. In embryos of from 7 to 

 10 mm. long these reach the wall of the pronephric tubule. 

 Running from one to another (in one case over three successive 

 segments) were found structures which may be interpreted as 

 glomi. These have also been seen by van Wyhe, who says 

 (p. 480) : " Es ist klein, sehr verganglich und besteht aus einem 

 gefassfiihrenden Strang, der von der dorsalen Lippe eines 

 Ostiums in schrager Richtung zur ventralen zieht. Der Strang 

 erstreckt sich in seiner 

 Mitte ringsum vom 

 Peritonealepithel be- 

 kleidet, frei durch die 

 Leibeshohle, und ist 

 an beiden Enden be- 

 festigt." Similar rudi- 

 ment al glomi were 

 found on the right side, 

 although the presence 

 of the " Darmgefasse " 

 modifies their position. 

 The objection made by 

 Rabl to this interpretation does not seem to be tenable in the 

 case of a rudimental organ, where one does not expect to find 

 the structure perfect or complete. 



In a number of the embryos the primitive ova containing 



FIG. 7. 



