7 i8 



AMNIOTA. 



the Ichthyopsida. In connection with its anterior extremity 

 certain structures have been found in the Fowl, which are 

 probably, on grounds to be hereafter stated, homologous with 

 the pronephros (Balfour and Sedgwick). 



The pronephros, as I shall call it, consists of a slightly 

 convoluted longitudinal canal with three or more peritoneal 

 openings. In the earliest condition, it consists of three successive 

 open involutions of the peritoneal epithelium, connected together 

 by more or less well-defined ridge-like thickenings of the 

 epithelium. It takes its origin from the layer of thickened 

 peritoneal epithelium situated near the dorsal angle of the body 

 cavity, and is situated some considerable distance behind the 

 front end of the Wolfifian duct. 



In a slightly later stage the ridges connecting the grooves 

 become partially constricted off from the peritoneal epithelium, 



FIG. 405. SECTION OF THE WOLFFIAN BODY DEVELOPING PRONEPHROS AND 

 GENITAL GLAND OF THE FOURTH DAY. (After Waldeyer.) Magnified 160 times. 

 m. mesentery; Z. somatopleure ; a', portion of the germinal epithelium from 

 which the involution (2) to form the pronephros (anterior part of Miillerian duct) takes 

 place; a. thickened portion of the germinal epithelium in which the primitive 

 germinal cells C and o are lying ; E. modified mesoblast which will form the stroma 

 of the ovary ; WK. Wolffian body ; y. Wolffian duct. 



