226 T. H. Burcenn, 
embryos of Pristiurus melanostomus, but also gave information with 
regard to Scylliwm, Raja clavata etc. 
Van WisHE distinguishes between pro- and mesonephros in the 
following way: I 
1. the former arises at the same time as the differentiation of 
its duct, the latter after its duct: 
2. the pronephros in all Vertebrates arises as segmented evag- 
inations of the somatopleure only, whereas the mesonephros arises 
from both somatopleure and splanchnopleure; 
3. the pronephrie duct constantly arises in connection with the 
pronephros, whilst the mesonephros gains a connection with the 
pronephric duct secondarily; 
4. the mesonephros possesses malpighian bodies, the pronephros 
does not: the “glomus” is not homologous with the glomeruli because 
it is a projection into the metacoelom (general coelom), not into the 
mesocoelom (nephrocoel). 
Van Wune divided the mesoderm into epimere (protovertebra), 
mesomere (nephrotome or Ursegmentstiel) and hypomere (lateral 
plate region). It is important to observe that van Wie realized 
that the upper part of the hypomere participates in the formation 
of the pronephros, although he also believed that this region was 
metameric. Thus he wrote: “Da nun der Pronephros, wie spätere 
Entwicklungsstadien zeigen, ein Produkt der Seitenplatte ist, wäh- 
rend der unmittelbar dorsal davon liegende Teil des Mesoderms zur 
Mittelplatte gehört, ist die Segmentierung des Mesoderms bei Sela- 
chiern also nicht auf die Myotomenplatte beschränkt, sondern er- 
streckt sich auch auf die Mittelplatte und den dorsalen Teil der 
Seitenplatte”, although there does not appear to be any evidence 
for this last statement if one may judge from his figures. Van WIJHE 
noticed the structures which RATHkE had called “Segmentalbläschen”, 
but said that they are transitory and take no part in the formation 
of the Selachian excretory system. He considered that the pro- 
nephric rudiment arises as a row of successive thickenings or out- 
erowths of the ventral portion of the somatic layer of the lower 
part of the nephrotome (?), which are directed towards the ectoderm 
but do not fuse with this layer; each thickening is a rudiment of 
a pronephrie tubule; since the tubules succeed one another so closely, 
they soon produce a Vornierenwulst (Solid cell-knob of BALFOUR). 
Van Wine said that the Sammelrohr (Sammelgang) is, from the 
first developed along with the tubules from the mesoderm, and that 
