162 THE FOURTH DAT. [CHAP. 



hind limbs ; but the duct itself is carried on still further 

 back. 



Towards the hind end of the embryo, the projection of 

 the intermediate cell-mass spoken of above becomes smaller 

 and smaller, and the Wolffian duct is thus brought nearer to 

 the splanchnopleure, and in the region of the hind-gut comes 

 to lie close to the walls of the alimentary canal. On the fourth 

 day, the two ducts meet and open into two horns, into which 

 the side-walls of the recently formed cloaca are at that time 

 produced, one on either side. 



As we shall afterwards see, the duct of the permanent 

 kidneys and Miiller's duct also fall into these two horns of 

 the cloaca. 



The Wolffian bodies thus constituted perform the offices 

 of kidneys for the greater part of embryonic life. In 

 the chick they disappear before birth ; but in most of the 

 Ichthyopsida they remain for life as the permanent kidneys. 



18. Near the end of the fourth day, on the outer surface 

 of the projection formed by the Wolffian body a furrow is 

 formed immediately below the Wolffian duct by an involution 

 of the germinal epithelium. This furrow, which is shewn at 

 M.d in Fig. 47, deepens, and its walls arch over and unite. 

 In this way a tube is formed, which separates from the 

 germinal epithelium in the same way that the neural tube 

 separated from the external epiblast. It is known as the 

 Duct of Milller ; of its function we shall speak later on. 



This account of the origin of Miiller's duct is due to Waldeyer {loc. cit.), 

 whose observations have been confirmed by subsequent inquirers. An exami- 

 nation of our own sections leads us to the same conclusions. 



Dr Sernoff (Centralhlatt fur Med. Wiss. 27 Jun. 1874) agrees with 

 Bomhaupt {Untersuchung ilher die Entivickelung des Urino-gemtalsy stems beim 

 Huhnchen) in considermg that the duct of Miiller is formed by a simple invo- 

 lution from the pleuroperitoneal cavity which grows backwards in the meso- 

 blast between the Woliiian duct and the germinal epithelium ; and thinks that 

 Waldeyer is in error in supposing the involution to be in the form of an elon- 

 gated furrow. This divergence of opinion is not of great importance compared 

 with the point on which both observers are in agreement, viz. that the duct of 

 Miiller is formed by an involution of the germinal epithelium from the pleuro- 

 peritoneal cavity. 



The formation of the duct of Miiller takes place from 

 before, backwards ; but near the hind end of the embryo, 

 where the germinal epithelium is deficient, the groove to 

 form the duct becomes an involution which, at first solid 



