THE LATER DEVELOPMENT OF THE CHICK 361 



solid rod, gradually becoming tubular, and reaches the cloaca 

 during the seventh day, although it does not acquire an opening 

 into the cloaca during embryonic life, indeed not until the fowl 

 is about six months old. The duct becomes surrounded by a 

 thick coat of mesenchyme cells and appears as a ridge on the 

 surface of the mesonephros. 



After the eighth day, in the male both Miillerian ducts and 

 in the female the right duct, cease to develop and immediately 

 begin a series of degenerative changes. In the female the left 

 duct continues to enlarge, and as the mesonephros disappears, 

 it remains as a conspicuous organ, attached to the dorsal body 

 wall by a double fold of peritoneum, the mesovarium. Further 

 differentiation into the regions of the adult oviduct already 

 described, begins before the end of the second week of 

 incubation. 



B. THE GONADS 



The early development of the gonads is alike in both sexes, 

 and it is not until the end of the first week that the sexes can be 

 distinguished. This early period is known as the indifferent 

 period. The gonads appear on the fourth day, as longitudinal 

 bands of thickened peritoneal epithelium, along the dorsal wall 

 of the body cavity, between the mesonephros and the attach- 

 ment of the mesentery. This band of " germinal epithelium" 

 later appears on the inner surface of the mesonephros, on 

 account of the enlargement of this organ. The germinal epithe- 

 lia develop symmetrically and extend through the posterior 

 half or third of the mesonephric region. 



The peritoneal cells of the so-called "germinal epithelium" 

 are apparently not to be regarded as the true germ cells. As 

 in other groups, the primordial germ cells are differentiated very 

 early in development, and migrate into this peritoneal or germi- 

 nal epithelium, where they begin to multiply (Fig. 142). The 

 mesenchyme cells of the mesonephros, beneath the peritoneum, 

 become added to the developing gonad and later form its stroma 

 or connective tissues. 



