VIII 



IX ^ 



X 



The pronephros of Chrysemj^s marginata. 29 



4,7 A nephrostome opening- into coelom in this section. 

 The tubule passing from it does not reach the 

 pronephric duct but ends blindly in 4,10. This tubule 

 is apparently degenerating. 



4.12) Nephrostome opening into coelom alongside the nephro- 



4.13) tome. 

 The tubule opens into the pronephric duct in section 



4,14. This tubule shows distinct evidence of a 

 further nephrostomic connection with the nephro- 

 tome in sections 4,15 and 4,16. 

 4,201 A nephrostome opens into lower end of nephrotome. 

 4,21/ A tubule passes up to join with the duct at 4,22; this 

 tubule opens into the lower end of the nephrotome 

 in 4,24 and 4,25. 

 At 4,28 the nephrotome region becomes enlarged and 

 lies close alongside the pronephric duct. It is 

 difficult at this stage to say how many communi- 

 cations there are with the duct, but they are prob- 

 ably present in sections 4,32 (?), 4,33, 4,34 and por- 

 sibly also 4,36. 



Behind 4,37 the nephrotome region appears as a hollow thick- 

 walled structure connected with both the coelom and protovertebra 

 by scattered mesenchyme cells. There is no trace of a com- 

 munication between duct and nephrotome behind section 4,37. 

 Further back the solid nephrotome gradually becomes joined with 

 protovertebrae and coelom by a double layer of mesoderm. The pro- 

 nephric duct ends at 7,22. There can be no doubt that it is formed 

 from the mesoderm posteriorly, for it can be seen in the process of 

 separating oif. 



Plate 2 Figs. 21, 22 and 23, show the gradation undergone in 

 passing from segments XI, XII and XIII to the region where the 

 pronephric duct ends (Fig. 23). 



The condition of the kidney on the right side. 

 Vascular glomus extending from 3,39 — 4,5. 



43 1 



^'^ I Nephrostome opening into coelom. 



^ 1 The tubule is rudimentary however and not connected 



with the pronephric duct. 

 4,10 Small nephrostome opening into coelom and nephro- 

 tome simultaneously. 



