130 



l']x('rot()r\- Orii'iins in Bdellostoina Stoiiti 



paper. Tlio ])r()iu'|)liros on the ri^lit side is in segments thiitv-two and 

 thirty-three. There are in all twenty-one tuhnles, and of these the first 

 three have no connection with the segmental dnct, while the last three 

 can be traced to a connection with Malpighian corpuscles, and must 

 therefore represent closed tubules. On the left side the pronephros is 

 in segments thirty-three and thirty-four, and there are here nineteen 

 lubules instead of twenty-one. The first two are not connected with the 

 duct, and two instead of three reisresent closed tubules. If the tubules 

 were arranged segmentally, the first would be in segment thirteen on 

 the right side and sixteen on the left, while the anterior closed tubule 

 would be in segment thirty-one on the right and thirty-three on the left. 

 On both sides the pronephros is connected with the mesonephros by the 

 segmental duct, and in neither case does it occupy the full width of two 

 seo-ments. 



Wb.v 



Fig. 31. 



Fig. 30. — Sagittal section through the pronephn.s showing several open tubules, all 

 but two of which are cut in longitudinal section. In this embryo the limits of the 

 pronephros are definitely established. hCj blood corpuscles ; d, segmental duct ; *, tubules. 



Fig. 31. — Transverse section through the pronephros of the oldest embryo studied, 

 showing one of the open tubules, t, in the process of branching, bv, blood-vessels; f/l, 

 glomerulus. 



Fig. 30 represents a sagittal section through the left pronephros. 

 Several tubules lie in about the same plane and are cut longitudinally. 

 Their connection with the segmental duct is shown. Two are cut trans- 

 versely. Farther on in the series these also join the duct. The section 

 is not near enough the median line to show an)' of the glomeruli. 



In the last embr3'o to be considered, which is some little older than 

 the embryo C, the exact position of the pronephros could not be deter- 

 mined, owing to an injury farther forward, but it is located in the 



