EXCRETORY ORGANS. 



591 



similar to those of the Reptilia, which foi-m the rudiments of the segmental 

 tubes. 



Secondary and tertiary segmental tubules are formed in the Chick, on 

 the dorsal side of the primary tubules, 

 as direct ditferentiations of the meso- 

 blast. They open independently into 

 the Wolffian duct. 



In Mammalia the segmental tubules 

 (Egli) are formed as solid masses in the 

 same situation as in Birds and Rei)tiles. 

 It is not known whether they are united 

 with the peritoneal epithelium. They 

 soon become oval vesicles, which de- 

 velop into comj)lete tubules in the 

 manner already indicated. 



After the establishment of the 

 Wolffian body there is formed in 

 both sexes in all the Amniota a 

 duct, which in the female becomes 

 the oviduct, but which is fimction- 

 less and disappears more or less com- 

 pletely in the male. This duct, in 

 spite of certain peculiarities in its 

 development, is without doubt ho- 

 mologous with the Miillerian duct 

 of the Ichthyopsida. In cimuection with its anterior extremity cer- 

 tain 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 con- 

 voluted longitudinal canal with three or more peritoneal openings 

 In the earliest condition, it consists of three successive open in- 



Fio. 403. Section through the 



EXTERNAL GLOMERULUS OF ONE OF THE 

 ANTERIOR SEGMENTAL TUBES OF AN EM- 

 BRYO Chick of about 100 h. 



gl. glomerulus ; ge. peritoneal epi- 

 thelium; Wd. Wolf36.an duct; uo. 

 aorta; me. mesentery. The segmental 

 tube, and the connection between the 

 external and internal parts of the glo- 

 merulus are not shewn in this figure. 



Fig. 404. Sections shewing two of the peritoneal invaginations which give 



RISE TO THE ANTERIOR PART OF THE MlJLLERIAN DUCT (PRONEPHROS). (After Balfour 



and Sedgwick.) 



A is the 11th section of the series. 

 B ,, loth 

 C ,, 18th 

 (ir2. second groove ; |/r.S. third groove; r2. second ridge; icd. Wolffian duct. 



