88 T. H. BURLEND, 



Fig. 18. Embryo H (sectian 5,12). This, and the next two figs, 

 represent successive sections through the region just behind protovertebra 

 XIII. Note the duct. 



Fig. 19. Embryo H (section 5,13). The pronephric duct ends in 

 this section on the right side. Notice its condition — a few scattered cells. 



Fig. 20. Embryo H (section 5,14). This section passes just behind 

 the pronephric duct on the right side. » 



Fig. 21. Embryo I (section 5,7). Passes through the region of 

 protovertebra XI. The duct, and the lower swollen portion of the 

 nephrotome are more deeply stained than the surrounding tissue. 



Fig. 22. Embryo I (section 6,39). Shows the condition in this 

 embryo in the region of segment XVIII (left side) and behind segment 

 XVIII (right side). The protovertebrae are in connection with the lateral 

 plate mesoderm. 



Fig. 23. Embryo I (section 7,21). This figure shows very well 

 the condition of the hinder end of the left pronephric duct as a few 

 scattered cells apparently proliferated from the mesoderm. 



Fig, 24. Embryo L (section 8,30). Figs. 24, 25 and 26 represent 

 sections each separated from the one preceding by one section only. They 

 show the condition in the region where the pronephric duct fuses with 

 the gut wall. 



Fig. 25. Embryo L (section 8,32). The duct is fusing with the gut 

 wall on both sides. The mesoderm and endoderm are also fused in the 

 ventral region of this section. 



Fig. 26. Embryo L (section 8,34). Notice that in figs. 24, 25 and 26 

 there is no connection whatsoever between ectoderm and pronephric duct. 

 The ectodermal groove on the outer side of the duct is well shown. 



Figs. 27 — 53 are photographs of sections through Embryo M. They 

 illustrate the structure and relations of pronephric and mesonephric 

 tubules. 



Fig. 27. (Section 4,13.) Shows glomus on each side. Also the 1st 

 pronephric tubule on the embi-yo's left side. 



Fig. 28. (Section 4,14.) Shows the nephrostome of the 1st pro- 

 nephric tubule on the embryo's left side : also the subdivided glomus on 

 each side. 



Fig. 29. (Section 4,15.) An aortic branch to the right glomus is 

 visible in this figure. 



Fig. 30. (Section 4,31.) Figs. 30, 31, 32, 33, 34 and 35 are 

 from sections passing through segment VIII or IX. A pronephric tubule 

 is obliquely cut on each side in figure 30. 



Fig. 31. (Section 4,32.) A nephrostome is cut through on the 

 left side. 



Fig. 32. (Section 4,33.) Nephrostome cut through on left side, pro- 

 nephric duct and glomus visible on the embryo's right side. 



