V THE DEVELOPMENT OF THE FROG 115 



later open into the coelom by funnel-shaped ciliated mouths 

 or nephrostomes, but the latter soon lose their connection 

 with the tubules and acquire secondarily an opening into 

 the branches of the renal veins in the ventral part of the 

 kidney. The tubules increase to a very large number and 

 become richly supplied with blood vessels ; they form with 

 the connective tissue which binds them together a compact 

 mass which assumes the form of the kidney of the adult. 

 The segmental or pronephric duct which served as the out- 

 let of the pronephros is worked in to form the Wolffian duct 

 or ureter of the adult. The Miillerian duct was formerly 

 supposed to arise by a splitting of the segmental duct, but 

 according to MacBride/ Marshall,- Gemmill,'' and more 

 recently Hall,^ it develops quite independently of that 

 structure. 



The reproductive organs first appear as ridges of the 

 peritoneum near the base of the mesentery (Marshall). 

 As the genital ridges increase in size they become con- 

 stricted at their points of attachment, and finally hang 

 supported by a peritoneal membrane. In the male the 

 testis becomes connected with tubes which grow out of the 

 renal tubules and form the vasa efferentia. The genital 

 ridges' in the two sexes have a similar appearance until near 

 the close of larval life, when those of the female undergo a 

 much more rapid growth. 



The beginning of the vertebral column is represented by the 

 notochord, but this structure forms but a relatively small por- 

 tion of the backbone of the adult frog. Loose mesodermic 

 cells, or mesenchyme, produced from the periphery of the 



1 MacBride, Quart. Jour. Mic. .Sci., Vol. 33, 1892. 



2 Marshall, " Vertebrate Embryology." 



8 Gemmill, Arch. f. Anat. it. P/iys.. Phys. Abth., 1897. 

 * Hall, Dull. A/us. Comp. Zool. Harvard, Vol. 45, 1904. 



