The pronephros of Scyllium canicula. 237 
separate at 2,97 and ends at 3,56. The pronephros therefore extends 
over nearly the whole of segment VII, the whole of VIII, and the 
anterior region of IX; the duct extends as far back as the front 
end of XVII, there are traces of pronephric arteries between segments 
VI and VII, and at the front and hind ends of segment VIII. 
On the right side the first indication of the pronephros appears 
in section 2,80; the duct becomes distinct at 2,97, and ends at 3,76. 
Thus the pronephros occurs in segments VII, VIII, and the anterior 
region of IX: the duct has grown back as far as the beginning of 
XX. This is the only case in my embryos in which the duct on the left 
side is shorter than its fellow on the right side. Further examination 
of this embryo pointed to an irregular development of the duct on the 
left side, since it undoubtedly merged into the mesoderm posteriorly, in- 
stead of approximating in position to the ectoderm as in other cases. 
On the right side there is a small pronephric artery (in sects. 2,81 
and 2,82, viz. between VI and VII), and a much larger artery in 
sections 2,87, 2,88 and 2,89, viz. between VII and VIII. There may 
also be the remains of a branch in sect. 2,95. 
I examined this embryo, as the others. in order to test the 
statement of Rickert and Ragz that the pronephros arises as 
metameric outgrowths from the mesoderm. Such a statement would 
require that intersegmental connections of somatic mesoderm with 
the pronephros are absent, also intersegmental nephrostomes. As I 
have elsewhere stated, there was equally as much evidence that the 
pronephros is formed from intersegmental structures as from seg- 
mental ones, and furthermore that the so-called “ostia” (potential 
nephrostomes of RABL) occur as frequently in the region between 
two segments as in the segments themselves. Again, there 
is no justification for speaking of tubules or of nephrostomes 
in any of the stages of Scyllium canicula earlier than Embryo I, 
because differentiation into tubules and duct does not occur before 
this stage. An inspection of the serial sections in the pronephric 
region of Embryos B, D and F, see Plates 18—24, is sufficient to 
convince one that the pronephric rudiment of Scylliwm (just as in 
Chrysemys) is of the nature of a primitive kidney groove, unsegmented 
in origin, and arising from the somatic mesoderm of the lateral 
plate region. The groove-like ancestral condition seems to be a 
more satisfactory explanation of all the conditions observed in the 
development of the pronephros in every Vertebrate yet described. 
See Plate 24 Fig. 74. 
