84 WILLIAM MORTON WHEELER, 



Plate 1. 



Fig. 1. Stage 1. Cross-section through the middle of the body- 

 before the formation of somites and the formation of the pronephros or 

 its duct ; showing the somatic and splanchnic layers of the mesoderm, 

 distinct medially but passing over into the entoderm laterally. The 

 neural canal and the myocœle and splanchnocœle are indicated. 



Eig. 2 — 8. Stage 2. Seven successive cross-sections through two 

 somites in front of the middle of the trunk. Fig. 2 from a section 

 through the extreme anterior edge of the first somite. Fig. 3 before 

 the middle, Fig. 4 through the middle, and Fig. 5 through the posterior 

 portion of the same somite. Figs. 6, 7 and 8 pass through the second 

 somite, the first through its anterior portion, the second through the 

 middle, and the last through its extreme posterior portion. These 

 sections show metamerism not onlj^ in the somite proper but in the 

 lateral mesoderm which contains a dilatation of the cœlom [nephro- 

 coele (cœ)]. 



Fig. 9. S t a g e 3. Transverse section through the region of the 

 second pronephric tubule which is indicated only by an enlargement of 

 the ccelom (cœ) and a slight thickening of the somatic wall of this 

 cavity (pron). In this embryo there were three of these openings 

 (nephrocoeles) in three successive segments. 



Plate 2. 



Fig. 10 — 24. Stage 3. Fifteen successive "cuneate" sections. 

 Showing the duct ainsing from small but distinct tubules. Fig. 10 begins 

 just back of the pronephric segments. The communication of the 

 tubules with the coelom is shown in Figs. 10, 17 and 24. The other 

 sections show the aborting tubules separated more or less from the 

 underlying somatic laj'-er. One section has been omitted between 

 Figs. 16 and 17. 



Fig. 25. Stage 2. Frontal sections tlirough the medullary tube 

 (w), showing the segmentation of the mesoderm. It is through two 

 successive somites such as those here figured that the sections repre- 

 sented in Figs. 2 — 8, Plate 1 pass. The constrictions at separate the 

 myotome with its enclosed myocœle from the nephrotome with the 

 enclosed nephrocoele which is also distinctly segmented. 



Fig. 26. Stage 3. Frontal sections showing the segmental origin 

 of the six pronephric tubules, and their tendency to occupy less space 

 then six segments. 



Fig. 27. Stage 3. Sagittal section through an embryo from the 

 lot represented in the preceding figure. Here, too, six pronephric tubules 

 are shown together with the formation of the collecting duct (cd) and 

 the anterior end of the pronephric duct (d). 



