Development of the Excretory Organs of IMellostomii stouti Lockington. 219 



In Bdellostoma, in statue A, there is an independent segmental 

 unlage at the anterior end of the excretory system, which later forms 

 a tubule, and which becomes connected with the following part of 

 the system, the connecting part forming the anläge of the segmental 

 duct. Through several segments caudalwards, we have the same 

 conditions, except that the anläge of the duct is already formed, but 

 even here there is one exception. There can be absolutely no doubt 

 that here the union between duct and tubules is primary and not 

 secondary, and later stages show that the lumen of the duct is here 

 formed in continuity with the lumina of the tubules. From this I 

 conclude that the anterior part of the excretory system of BdeUostoma 

 develops like the pronei)hros of other vertebrates, and I cannot see 

 how the conclusion can be avoided, except by doubting the correctness 

 of the observations. 



But through the greater part of the system the duct and tubules 

 are already formed, and the question arises, did this part develop as 

 a pronephros also, or were the duct and tubules formed independently 

 of each other, and only secondarily became connected, as is the case 

 in the development of a mesonephros? I have been unable to find 

 anything which indicates a secondary connection between duct and 

 tubules, while there are at least four strong reasons for thinking they 

 have developed in continuity: 1) The transition between the anterior 

 part, where both duct and tubules are in the earliest stage of develop- 

 ment, and the posterior part, where they are in a more advanced 

 stage, is a perfectly gradual one. If one portion developed in one 

 way, and the other portion in a different way, we should expect to 

 find at least some slight line of demarcation between them , but 

 such is not the case. 2) In one embryo at least, on one side, in the 

 region corresponding to the position of the posterior mesonephric 

 tubules of tlie adult, there are places where the duct is just in the 

 process of being constricted off" from the coelomic epithelium. Here 

 the duct and tubules must have developed in continuity, or, may it 

 not be said, are developing in continuity. It would naturally follow, 

 that in the part between here and the anterior end the duct and 

 tubules have also developed in continuity. 3) In stage B, in a part 

 of the region anterior to the position of the pronephros of the adult, 

 the stage of development of the duct and tubules is essentially the 

 same as in the posterior region in stage A, but we know from stage 

 A that in this region the duct and tubules develoi)ed in continuity, 

 and it seems fair to suppose that a still younger embryo would show 



