264 T. H. Burtexp, 
Fig. 66 (Section 2,54). In this and the preceding section the third 
right pronephric artery is seen passing off from the aorta. 
Fig. 67 (Section 2,55). The pronephros in the region between 
segments VIII and IX is here seen. 
Fig. 68 (Section 2,56). Notice that the glomus in this and neigh- 
bouring sections causes the splanchnic mesoderm immediately above it to 
become contiguous with the somatic mesoderm, so that the coelom between 
these layers is practically absent. 
Fig. 69 (Section 2,57). From an examination of this and other 
sections it becomes indisputable that the pronephros is entirely mesodermal 
#in origin. There is no ectodermal participation, such as RÜCKERT 
averred. 
Fig. 70 (Section 2,58). The glomus is now decreasing in size in 
the posterior region of the pronephros. 
Fig. 71 (Section 2,59). The connection between the distal part of 
the pronephros and the somatic mesoderm is becoming narrower on the 
left side of the figure. 
Fig. 72 (Section 2,60). This section passes through somite IX on 
each side. 
Plate 24. 
Fig. 73 (Section 2,61). The duct is now free from the somatic 
mesoderm on the left side of this figure: it is also quite free from the 
ectoderm. 
Fig. 74 (Section 2,83 of embryo G). Shows the pronephric rudiment 
in the region of protovertebra VII. This section apparently corresponds 
exactly with that shown in Fig. 52 (for embryo F). There is no doubt 
that the presence of the vascular system on the right side of the embryo 
tends to render the relations of the rudiment on this side less simple than 
on the left side. 
Figs. 75—85 are photographs of successive sections of embryo I. 
Fig. 75 (Section 2,59). The pronephric rudiment is in the form of 
a groove in the lateral plate region. Notice the large vitelline artery. 
Fig. 76 (Section 2,60). On the left side there is visible what appears 
to be an abortive attempt on the part of the wall of the vitelline artery 
to fuse with the lip of the pronephrie groove. 
Fig. 77 (Section 2,61). The groove is closed in this and the 
preceding section, but the duct produced has a solid connection with the 
coelomic lining. 
Fig. 78 (Section 2,62). The pronephric groove is about to open to 
the coelom, but into a portion of the latter which now has a nephrocoelic 
nature. The groove on the right side of the figure is still open. 
