102 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



duct being the equivalent to a series, so to speak, of Ovi- 

 ducts. 



I pointed out before that the mode of development of the 

 Oviduct could only be considered as a modification of a simple 

 involution from the pleuro-peritoneal cavity. Its development, 

 both in the Birds and in the Batrachians as an involution, still 

 more conclusively proves the truth of this view. 



The explanation of its first appearing as a solid rod of cells 

 which keeps close to the epiblast is, I am inclined to think, 

 the following. Since the Oviduct had to grow a long way 

 backwards from its primitive point of involution, it was clearly 

 advantageous for it not to bore its way through the mesoblast 

 of the intermediate cell mass, but to pass between this and the 

 epiblast. This modification having been adopted, was followed 

 by the knob forming the origin of the duct coming to be placed 

 at the outside of the intermediate cell mass rather than close 

 to the pleuro-peritoneal cavity, a change which necessitated 

 the mode of development by an involution being dropped and 

 the solid mode of development substituted for it, a lumen being 

 only subsequently acquired. 



In support of the modification in the development being due 

 to this cause is the fact that in Birds a similar modification has 

 taken place with the Wolffian duct. The Wolffian duct there 

 arises differently from its mode of development in all the lower 

 vertebrates as a solid rod close to the epiblast 1 , instead of as an 

 involution. 



If the above explanation about the Oviduct be correct, then 

 it is clear that similar causes have produced a similar modifica- 

 tion in development (only with a different organ) in Birds ; while, 

 at the same time, the primitive mode of origin of the Oviduct 

 (Muller's duct) has been retained by them. 



The Oviduct, then, may be considered as arising by an invo- 

 lution from the pleuro-peritoneal cavity. 



The Wolffian duct arises by a series of such involutions, 

 all of which are behind (nearer the tail) the involution to form 

 the Oviduct. 



1 If Romili's observations (Archives fiir Mikr. Anatoin. Vol. IX. p. 200) are 

 correct, then the ordinary view of the Wolffian duct arising in Birds as a solid rod at 

 the outer corner of the prutovertebnv will have to be abandoned. 



