234 Vertebrate Embryology 



the end of the fourth day, three pit-like involutions of 

 the peritoneal epithelium appear, one behind another, 

 close to the outer side of the Wolffian duct, and three or 

 four somites behind its anterior end (Fig. 74, M). A 

 ridge-like thickening of the peritoneal epithelium con- 

 nects the three pits of each side with one another, and 

 grows backwards behind the third pit as a solid rod of 

 cells, lying along the outer side of the Wolffian duct, and 

 very close to this. This rod soon becomes tubular, 

 ending blindly behind, but opening in front into the 

 body cavity through the three pits. These three pits 

 form the head-kidney of the chicken embryo, and the tube 

 into which they open is the commencement of the 

 Mullerian duct. Towards the end of the fifth day the 

 two hinder pits close up and disappear. The anterior 

 pit persists, and forms the peritoneal opening of the 

 Mullerian duct or oviduct. The Mullerian duct itself 

 grows rapidly backwards ; it lies in close contact with 

 the outer wall of the Wolffian duct, and in its hinder 

 part appears to be formed from cells derived from the 

 wall of the Wolffian duct. About the end of the sixth 

 day, the Mullerian duct has grown backwards as far as 

 the cloaca. It remains blind at its hinder end in the 

 male, but in the female opens, at a later stage, into the 

 cloaca" ' 



and becomes the oviduct. It is only on the left 

 side of the female, however, that the Miiller- 

 ian duct becomes the oviduct : on the right 

 side it practically disappears, though a trace 

 of it may persist in the adult. In the male 



' Marshall. 



