278 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



the higher vertebrates that the ostium of an undoubted Mlillerian 

 duet is liable to undergo secondary shifting into such a position. 

 Again the fact that the opening lies on the mesial side of -the ovary 

 is adduced as an argument in the same sense but in this case we 

 have definite embryological evidence from Polypterus (Budgett, 1902) 

 that this position is secondary, the early rudiment of the duct lying 

 external to the ovary and immediately ventral to the Wolffian duct 

 as is the case with a typical Mlillerian duct. Consequently there 

 is no sufficient reason to doubt that these oviducts with open ostia 

 in ganoids are really Mlillerian ducts. 



Phylogeny of Teleostean Oviduct. — The facts of development 

 show clearly that the main part of the Teleostean oviduct is of the 

 same morphological nature as the ovary with which it is continuous. 

 It arises from the hinder part of the primitive ovary which has 

 become sterile and assumed a merely conducting function. 



The main difficulty connected with the morphology of the organ 

 is that of accounting for the joining up of the part of the oviduct 

 of ovarian origin with the cloaca or exterior. Balfour suggested 

 that this had come about by the oviduct becoming fused with the 

 lips of the " abdominal pores." As an objection to this was adduced 

 the observation by Hyrtl that in Mormyrus abdominal pores exist 

 along with oviducts. This objection disappears, however, if we 

 remember that in Balfour's time there were confused together under 

 the same name two different types of aperture — true abdominal 

 pores 'and genital pores. Substituting genital pores for abdominal 

 Balfour's view seems still the most feasible. The probability seems 

 to be that the main steps in the evolution of the Teleostean oviduct 

 were as follows : — 



(1) The primitive oviduct or Mlillerian duct underwent gradual 

 atrophy becoming gradually shorter 1 until eventually nothing was 

 left but its external opening — the genital pore. This process would 

 doubtless be correlated with the loss of its glandular function and 

 this in turn may have been connected either with the adoption of 

 pelagic spawning, in which special tertiary investments for the eggs 

 were no longer required, or with a special development of primary 

 envelopes within the group. A stage was thus reached which is 

 represented by the condition in Salmo. Of course we do not know 

 whether Salmo has retained this condition or has reverted to it ; 

 the latter is more probable. 



(2) The portion of splanchnocoele along the ovigerous surface 

 became enclosed so as to form a cavity which served to conduct the 

 shed ova back into the neighbourhood of the genital pore. Anteriorly 

 the ovarian surface abutting on this cavity remained fertile, while 

 posteriorly it became sterile, so that the posterior portion of the 

 cavity performed merely a- conducting function (oviduct). 



(3) The lips bounding the posterior end of the oviduct from 



1 We ma)- see early stages in this process illustrated by the Ganoids Amia and 

 Acipenser. 



