PRONEPHROS IN PETROMYZON. 



139 



7) . In the anterior region, the segmental duct has every appearance 

 of having been formed in the same manner by the thickening of the 

 proximal margin of the lateral plates, as in the pronephros proper ; the 

 only difference is that the anlagen do not develop into tubules, but 

 unite with each other to form the duct. Owing to a large quantity of 

 yolkmass in the posterior region the process is here much delayed and 

 somewhat modified : it is brought about by the multiplication of a few 

 cells proliferated from the proximal margin of the somatic layer of the 

 lateral plates. I bave observed neither any trace of an epiblastic origin 

 of the duct nor any free growth of its posterior end*. In a much later 

 stage, the posterior extremities of the ducts open into the cloacal section 

 of the enteric canal. 



8) . From early stages, there is a complete blood supply in the pro- 

 nephros : the arterial blood comes from the dorsal aorta ; tbe 

 blood corpuscles are found scattered between the tubules, and are 

 afterwards transformed into two pairs of giorni, the anterior of which 

 soon atrophies. The venous blood is taken away by the cardinal veins 

 which drain the pronephros from early stages. 



From the facts mentioned above the following conclusions are 

 justified: Both the pronephric tubules and the segmental ducts are 

 purely organs of the lateral unsegemented portion of the mesoblast t ; 

 the somatic mesoblast as well as the other germinal layers have no 

 share in their formation. The anlagen of the tubules follow, from 

 their first appearance, the same segmental arrangement as the 

 mesoblastic somites. The maximum number of the pronephric tubules 

 formed is 6 pairs, of which the first, second, and sixth degenerate one 

 after the other. The persistent tubules are, therefore, the third, fourth, 

 aud fifth, of which the third pair is not so well developed as the next 



* I have observed a case in which mitotic cell divisions are taking place in that point 

 of the epiblast, where the segmental duct lies in close contact with it ; this subject will bo 

 explained in the full paper. 



t For convenience's sake I divide the mesoblast into two portions the somatic and 

 the lateral plates, and no more. 



