hall: mesonephros and mullerian duct in amphibia. 113 



peudent growth probably represents an extension cephalad of the process 

 whereby the posterior end of the Miillerian duct reached a separate 

 external opening. As suggested by Kip ('94, '94=^), one can imagine 

 this process as phylogenetically advancing cephalad until the posterior 

 prolongation of the ostial evagination reaches the cloaca without estab- 

 lishing any connection with the Wolffian duct. This stage seems, indeed, 

 to have been reached in the Anura.^ A connection between the two 

 ducts has been claimed, however, for members of every group of the 

 vertebrates which possess a Mullerian duct, with the exception of the 

 Reptilia. It may be noted, also, that the reptiles are the only remain- 

 ing group (since my observations on the Amphibia now exclude that 

 group) in which we have not some evidence that there is more than one 

 evagination as forerunner of the ostium abdominale.^ 



F. Recapitulation of the Development op the Mullerian Duct. 



Amhlystoma. 



The first trace of the fundament of the Mullerian duct appears, some 

 time before the degeneration of the gills and pronephros, in the shape of 

 a thickening of the peritoneal epithelium beneath the first nephrostome. 

 This thickening forms a band of crowded, cylindrical cells, which ex- 

 tends ventrad from the neplirostrome, but is soon forced to turn caudad 

 by the growth of a "shelf" of tissue which extends horizontally across 

 that portion of the body cavity which is dorsal to the fusion of the lung 

 with the lateral body-wall. This shelf thus divides the dorsal portion 

 of the body cavity into an upper chamber, the glomerular cavity, and a 

 lower chamber, the sub-glomerular cavity. 



Soon the thickened band extends from the first nephrostome back 

 along the dorsal surface of the shelf, then ventrad around its posterior 

 edge and forward again on its ventral surface. As it passes back beneath 

 the second nephrostome it is joined by a similar band from that organ. 



Immediately ventral to the first nephrostome there appears a local 

 accentuation of the thickening to form a small, thick disk. A similar one 

 then appears beneath the second nephrostome. A depression in the sur- 

 face of each of these disks results in the formation of an open pit, the 

 wall of which extends into the substance of the pronephros. 



1 Figures S and T (page 111) represent tlie conditions in Hyla and Rana, — a 

 total independence of Miillerian and Wolffian ducts. They also show, when com- 

 pared with Figure Q, the modifications undergone by nephrostomes and Miillerian 

 evaginations. 



2 Burger ('94, '94^) describes the duck as having as many as fifteen. 



