390 THE ORIGIN OF VERTEBRATES 
metanephric tubules are formed, and the former alone gives origin 
to a duct, which forms the basis for the generative and urinary 
ducts, and is called the segmental duct. The mesonephric tubules, 
called also the Wolffian body, open into this duct. 
In order to make the embryology of these excretory organs quite 
clear, I will make use of van Wijhe’s phraseology and also of his 
illustrations. He terms the whole celomic cavity the procelom, 
which is divisible into a ventral unsegmented part, the body-cavity 
or metacelom, and a dorsal segmented part, the somite. This latter 
part again is divided into a dorsal part—the epimere—and a part 
connecting the dorsal part with the body-cavity, to which therefore 
he gives the name of mesomere. 
The cavity of the epimere disappears, and its walls form the muscle 
and cutis plates of the body. The part which forms the muscles is 
known as the myotome, which separates off from the mesomere, leaving 
the latter as a blind sac—the mesocelom—communicating by a narrow 
passage with the body cavity or metacalom. At the same time, from 
the mesomere is formed the sclerotome, which gives rise to the skeletal 
tissues of the vertebre, etc., so that van Wijhe’s epimere and mesomere 
together correspond to the original term, protovertebra, or somite of 
Balfour; and when the myotome and sclerotome have separated 
off, there is still left the intermediate cell-mass of Balfour and 
Sedgwick, i.e. the sac-like mesoccele of van Wijhe, the walls of which 
give origin to the mesonephrotome or mesonephros. Further, accord- 
ing to van Wijhe, the dorsal part of the unsegmented metaccelom is 
itself segmented, but not, as in the case of the mesoccele, with respect 
to both splanchnopleuric and somatopleuric walls. The segmentation 
is manifest only on the somatopleuric side, and consists of a distinct 
series of hollow somatopleuric outgrowths, called by him hypomeres, 
which give rise to the pronephros and the segmental duct. 
Van Wijhe considers that the whole metaccelom was originally 
segmented, because in the lower vertebrates the segmentation reaches 
further ventral-wards, so that in Selachia the body-cavity is almost 
truly segmental. Also in the gill-region of Amphioxus the cavities 
which are homologous with the body-cavity arise segmentally. 
As is well known, Balfour and Semper were led, from their 
embryological researches, to compare the nephric organs of vertebrates 
with those of annelids, and, indeed, the nature of the vertebrate 
segmental excretory organs has always been the fact which has kept 
