224 EMBEYOLOGY OF THE LOWEE VEETEBEATES 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). 



Fig. 122. — Early stages in the development of the pronephros of HypogeopMs. Each 

 figure represents a' longitudinal section, so arranged as to pass outwards through the 

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

 1902, slightly simplified.) 



A, from an embryo with 15 mesoderm segments ; B, 12 segments ; C, 16 segments ; D, 27 segments. 

 a.n.d, archinephric.duct; jm, pronephric tubule. The Roman figures are placed in the nephrocoeles. 



The first signs of the pronephros make their appearance — in 

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

 bulgings outwards of the outer or somatic wall of the nephrotome of 

 segments IV and V. These outward bulgings are the rudiments 

 of the pronephric tubules. A third soon appears in segment VI 

 (Fig. 122, A, cf. also Fig. 123, A, pn). The three rudiments grow 

 actively in length pushing their way tailwards along the body just 

 external to the nephrotomes. They come to be in close contact 

 and presently fuse to form a rod-like structure (Fig. 122, B) which 

 continues to extend backwards towards the tail and becomes tubular 

 through developing a cavity secondarily in its interior. This, at 

 first solid, rod-like structure (Fig. 122, C, a.n.d) is the rudiment of 



