250 
Transactions of the Boyal Society of South Africa. 
are not, however, developed as simple outgrov^^ths, but have what may 
be called an endogenous " origin. Each arm is seen to lie in a little sac 
cy 
Fig. 8. — Schematic representation of the principal veins of the Xenopus tadpole 
{the pulmonary veins are omitted) from reconstructions of serial sections. PN = 
pronephros ; PND = pronephric duct ; MN = mesonephros. For rest of lettering 
see text. 
(Fig. 6), but my material was not young enough to determine whether 
this sac was formed as an independent invagination or as a separation 
