hall: MESONEPHKOS and MULLERIAX duct IX AMPHIBIA. 43 



Stage V it is impossible to decide. The more dorsnl portion of the 

 somatic layer of the epimer has also been converted into mesench^'me. 

 Of the two thinner walls of the somite, the lateral and the ventral, 

 there thus remain only the mesomer and a small adjacent part of the 

 somatic layer of the epimer (so'dnn. t., Fig. 7). The latter bounds the 

 ventral and lateral sides of the remnant of the epicoelom. 



To understand Figures 7, 8, and 9, it is necessary to bear in mind the 

 opisthocoelous character of the somites and their oblique position. Xot 

 only are they placed obliquely, but their ventro-lateral portion, with the 

 enclosed lumen, has been stretched backward, so to speak, until the 

 most posterior point of each somite lies well back on the next following. 

 Figure 8 represents a section through the middle of a somite ; Figure 7 

 shows one passing back of the middle of one and barely cutting the 

 anterior end of another (which would be seen above and to the right of 

 the myocomma, mj/cm.) ; the section represented in Figure 9 passes 

 through the posterior end of a somite. The overlapping of the somites 

 causes the mesomer to extend obliquely outward and backward, so that 

 in the anterior portion of the somite it lies some distance from the 

 duct and mesad of the postcardinal vein, while toward the posterior 

 end it lies against the duct (Fig. 9, ms'mer.). 



The point to which attention is to be especially directed is the appear- 

 ance of the mesomer, for it is in the mesomer and at this stacre that the 

 fundaments of the me-ionejihros are first discernible. These fundaments 

 arise in the embryo under consideration in the posterior fourth of each 

 mesomer from somite 9 to somite 18, inclusive. AYithout doubt the one 

 or two remaining somites anterior to the cloaca give rise in a similar 

 manner to mesonephric fundaments, as the blastema^ is seen to extend 

 in older larvae to the posterior end of the duct. This question could 

 not be decided definitely, as that part of the body in the particular 

 embryo described above was lost, and it was found that in other speci- 

 mens (and pi-esumably this would have been likewise true of that 

 specimen) the curving of the body renders all processes in the posterior 

 region obscure. 



The differentiation of the mesonephric fundaments may be described 

 as follows : the posterior fourth of each mesomer (Figs. 7, 9, ms'mer.) 

 distinguishes itself from the remaining portion (and from the entire 

 mesomer of somites anterior to the ninth) by three peculiarities : the 



1 The term " blastema " has been employed to designate the continuous cell-mass 

 from which the mesonephros arises in Amniota. As will be shown in the descrip- 

 tion of the next stage, the term applies equally well to Urodela. 



