WOLFFIAN DUCT AND BODY IN THE CHICK. 41 



than a trace of the hinder part should appear simaltaneouslj in 

 embryonic development with the anterior part. If the rest of 

 the mesonephros developed continuously with the duct and 

 simultaneously with the pronephros, then, on the above hypo- 

 thesis, we should not be able to distinguish a pronephros from 

 the hinder part ; and it is opposed to all our ideas of economy to 

 suppose that a rudiment ot the mesonephros should appear at 

 what phylogenetically would be the proper time, remaining over 

 as a rudiment in the larva, i. e. as a useless organ forming 

 merely a burden until it was wanted. 



It seems to me that we can only expect, at the very utmost, 

 to find a very small trace of the mesonephros in embryonic de- 

 velopment at what phylogenetically we should consider, on the 

 above hypothesis, to be the proper moment relative to the 

 pronephros. 



I have been examining the development of the segmental 

 duct in an Amphibian, the frog, to see if at the time of closure 

 of the groove of the segmental duct any trace of a discontinuous 

 closure such as we find in the head-kidney existed. If the 

 pronephros is merely the anterior part of a segmental organ 

 of which the mesonephros is the posterior part, and if 

 phylogeny is in any way repeated in the development of the 

 pronephros, we should expect to find that the discontinuous (seg- 

 mented, see above) closure of the pronephros would be repeated 

 behind, showing some traces at least of the openings of the 

 segmental duct and of the specialised part of the body cavity 

 which later forms the Wolffian tubule and contains the glome- 

 rulus. So far it cannot be said that my search has been from 

 my point of view successful. To get any evidence of what I was 

 searching for requires a very complete series of sections in a state 

 of preservation favorable for observation. The difficulties pre- 

 sented by the embryonic Amphibia in their early stages to such a 

 successful result are very great. In the first place they are 

 very brittle, and comparatively very few of the sections, even if 

 thick, can be mounted uninjured. Of these, very few, indeed, 

 can be obtained perfect, and. those so obtained are apparently 

 more difficult to see anything in than the thick ones. The cells 

 are full of yolk granules which seem to escape and obliterate the 

 outlines of the cells from the J?ight. 



While my results have not been such as to unable me to speak 

 with any confidence either one way or the other, yet on the 

 whole they have convinced me that a re-examination with a new 

 method of the development of the segmental duct in Amphibia, 

 &c., would repay the trouble. 



In the chick, on the other hand, the anterior part of tlie seg- 

 mental duct, for the space of five segments, develops exactly in 



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