224 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



ancestral methods than in the case of those Vertebrates (e.g. Elasmo- 

 branchii, Amniota) in which the organ is modified to the extent of 

 being reduced to a functionless rudiment. 



(2) Its histological texture is comparatively coarse and the 

 general structural arrangements in the embryo are so distinct as to 

 eliminate to a great extent risk of observational errors. 



(3) Its development has formed the subject of a particularly 

 careful and complete investigation (Brauer, 1902). 



and 



Fi'.. 122. -Karly stages in the development of the pronephros of HypogeCfhii, K:i-li 

 figure represents a longitudinal section, so arranged as to pass outwards through tin- 

 nephrotomes, cutting them across, and viewed from tin- dorsal side. (After Brauer, 

 1902, slightly simplified.) 



A, from an embryo with 15 mesoderm segments ; B, ]_> sr.mnrnts ; ('. li> segments ; 1). _'? segments. 

 .!<-hin'plirir duel ; /m, pronrpl.rir tubule. Tin- Human figures are placed 



The first signs of the pronephros make their appearance in 

 embryos with about 9 or 10 mesoderm segments in the form of 

 biil^iiiLrs (ui wards of the outer or somatic wall of the nepliroiimie of 

 segments IV and V. These outward 1 minings are the rudiments 

 f th'- proiK'plmc tubules. A third soon appears in segment VI 

 Fi'_ r . 1'JL'. A., of, also Fig. 123, A, pn). The three rudiments urn\\ 

 actively in length pushing their way tailwards along the body just 

 external t" the n&phrotomee, They emne 1<> he in close contact 

 and presently fuse to lrm a rod-liko structure (Fi-\ I--. 1'^ which 

 continues in extend backwards tu\\ards the tail and becomes tubular 

 Umni i, cavity secondarily in its interior. This, at 



first Holid, rod-like structure (Ki.LT. 1-U, i\ u.n.<l) is the rudiment d 



