1887.] Mr G. Brook on Epiblastic Origin of Duct. 369 
when, on the publication of Graf Spee’s researches on the guinea 
pig in 1884, the segmental (pronephric) duct was shown to have 
an epiblastic origin. Hensen, indeed, had noted the fact some 
years previously, but no notice had been taken of his discovery 
until Graf Spee called attention to it. Hensen has recently taken 
up the subject again, and Flemming has published a confirmatory 
account for the rabbit. Thus there appears no further room for 
doubting the epiblastic origin of the segmental duct in mammals. 
It does not necessarily follow that the whole excretory system has 
an epiblastic origin, but further information is required on the sub- 
ject. Towards the end of 1886 Van Wijhe demonstrated the 
epiblastic origin of the segmental duct in Elasmobranchs, and 
during the present year Yon Perenyi has announced that the 
epiblast plays a similar part in Rana and Lacerta. During the past 
few months I have been enabled to confirm Yon Perenyi’s researches 
so far as Rana is concerned, and have also found that, in regard to 
the formation of the segmental duct, Teleostean fishes, and probably 
also birds, are in agreement with other types. The epiblastic origin 
of the segmental duct is probably a feature common to the Yerte- 
brata generally. 
In the trout the segmental duct arises almost precisely in the 
manner described and figured by Flemming for the rabbit. In a 
twenty-seven days’ embryo the duct is well marked in the middle 
trank region, and thins out both anteriorly and posteriorly. An- 
teriorly the duct appears as a thickening of that portion of the 
surface epiblast overlying the intermediate cell mass ; that is to 
say, the segmental duct arises from that part of the epiblast dorsal 
to the portion of the mesoblast from which it was formerly supposed 
to be derived. Passing posteriorly the epiblastic thickening be- 
comes more and more important, and in the middle trunk region 
forms a large rounded mass of cells still partly attached to the 
epiblast, and situated between the vertebral plate and the lateral 
mesoblast. The lumen of the duct appears first as an irregular 
cavity, and later the whole mass loses its connection with the 
epiblast, and becomes pressed in amongst the “ intermediate cell 
mass ” during the formation of the lateral body folds. 
The origin of the segmental duct in birds does not appear to be 
quite as clear. Anteriorly, the epiblast covering the central nervous 
