THE WOLFFIAN- DUCT AND BODY. 117 



presently projects as a distinct vascular organ along the dorsal part of the peritoneal 

 cavity on either side of the mesentery. Subsequently another duct becomes deve- 

 loped along the outer side of the Wolffian body, along which it runs backwards to 

 open also into the cloaca : in front it communicates with the pleuroperitoneal cavity 

 by one or more funnel-shaped apertures (fig. 143, z\ This is the Miillerian duct, so 

 named after Johannes Miiller ; in some of the lower vertebrates it arises in common 

 with the Wolffian duct. From the lower end of each Wolffian duct a hollow pro- 

 trusion (fig. 125, C and D, N) grows upwards into a mass of mesoblast continuous 

 with that of the Wolffian body ; with the branches of this protrusion glomeruli and 

 convoluted tubes also become connected, and thus the permanent kidney (hind-kidney, 

 metanephros) is produced. Lastly, the ccelornic epithelium covering the inner side of 

 the Wolffian body becomes thickened (fig. 143, a), and within it are found larger 

 cells, from which the generative products in both sexes (ova and spermatozoa) arc 

 eventually derived. This epithelium is accordingly known as the germinal epithelium. 

 The duct of Miiller becomes in the female the oviduct or Fallopian tube ; in the 

 male it becomes atrophied. The Wolffian duct in the male becomes the epididymis 

 and vas deferens ; while the vasa efferentia and tubes of the rete testis are formed 

 as outgrowths from the Wolffian body ; in the female these parts have no permanent 

 function. 



The head kidney, although permanent and functional in fishes, is only a rudi- 

 mentary organ in the embryo of higher vertebrates, and soon disappears. The 

 Wolffian body is well developed in all vertebrates ; in fishes and amphibia it is an 

 important part of the permanent urinary apparatus, and also serves to carry away 

 the male sexual products (fig. 138). In higher vertebrates (amniota) it no longer 

 continues to perform excretory functions, but still supplies the efferent apparatus of 

 the testis. 



The details of the development of these parts may next be considered. 



The Wolffian duct and body. The commencement of the Wolffian duct is 

 seen at a very early period of development (second day in the chick, eighth day in 

 the rabbit) as a thickening of the intermediate cell-mass in the anterior region of 

 the trunk (fifth somite) (fig. 39, Wd). The outgrowth projects towards the epiblast, and 



p. 



Fig. 139. TRANSVERSE SECTION OF AN EMBRYO CHICK OP THIRTY-SIX HOURS, ^f (E. A. S.) 



n.c, medullary tube ; p, protovertebra ; ep, epiblast ; me, lateral mesoblast split into splanchnopleure 

 and somatopleure ; coe, pleuro-peritoneal cavity between them ; cce', cavity of protovertebra. continuous 

 on the right side with the lateral mesoblastic cleavage; W.d., Wolffian duct; W.b., mesoblast of 

 Wolffiau body ; ch, notochord. 



developes from before backwards ; a solid cord of mesoblast thus becomes formed, which 

 gradually becomes detached from the remainder of the intermediate cell-mass, lying 

 close to the epiblast (fig. 139, ung). Soon after it is thus formed, a lumen appears in 

 :it and extends both forwards and backwards. The posterior end, which is still solid, 

 is presently found to be attached to the epiblast, and apparently continues to grow 

 backwards along and at the expense -of the epiblast until it reaches the posterior end 



