250 T. H. Burtenp, 
betrachten, wo der Pronephros zuerst mit der Haut verschmitzt. 
Dieselbe liegt ungefähr lateral vom Hinterende der erwähnten 
Drüse”. — 
Rast says that the duct is entirely mesodermal, and grows by 
its own increase, owing to active karyokinesis (cf. Bazrour, 1878). 
GREGORY, 1892, ascribes a double origin to the duct: 
1. by a splitting from the ectoderm, 
2. by its own increase. 
I have noticed karyokinesis going on in the growing duct, but 
not of such a nature as to suggest that this is the means by which the 
duct grows backwards: although a close contiguity exists between 
the ectoderm and duct, yet my observations are quite in accordance 
with those of RABL in that he believed the duct to be entirely 
mesodermal in origin. The duct is quite distinct from the ectoderm 
near its posterior end and seems to merge into the mesoderm; the 
duct stains like the mesoderm, not like the ectoderm; there are 
many traces of a wandering of mesoderm cells from the somatic 
layer of the mesoderm to the duct as the latter grows caudalwards. 
Furthermore, in the case of my embryo G the duct on the left side 
appears to be abnormal in growth, since it only extends as far back 
as segment XVII, whereas on the right side the duct has grown 
back to segment XX. In this case the left pronephric duct clearly 
seems to end intersegmentally in the somatic mesoderm. 
Unlike Rickert for Torpedo, and RaBz for Pristiurus, I find 
that the right duct almost invariably lags behind the left in back- 
ward growth. It may be that Rası meant the right side of his 
figures, viz. the left side of his embryos, when speaking of this back- 
ward growth! 
Rickert stated that in Torpedo the duct separates from the 
ectoderm at the level of the cloaca and grows into the latter. 
VAN WIJHE said that the caudal end of the duct fused with the 
cloaca and the ectoderm. He found the duct still ending blindly in 
a 30 mm long Pristiurus embryo. Rast, however, found that in 
both male and female embryos of 25,3 mm the duct on each side 
ended in a papilla of the cloacal wall dorsally, but that it did not 
open into the cloaca. So far as I am able to gather from my sec- 
tions, the pronephric duct is derived from the mesoderm, and be- 
comes identical with this layer in the region of, or a little way in 
front of, the neurenteric canal. Thus there is no question of a structure 
— the duct — being derived from one germinal layer and later 
