THE REGION OF THE SPINAL CORD 389 
has arisen from the uniformly segmented simple longitudinal muscu- 
lature of the lowest fish, so, as we pass down the vertebrate phylum, 
we find more and more of a uniform segmental arrangement in the 
excretory organs. 
The origin of all these three separate segmentations may, in 
accordance with the phraseology of the day, be included in the 
one term—the origin of the spinal mesoblastic segments—z.ec. of the 
ccelomic cavities of the trunk-region and the structures derived from 
their walls. 
THE ORIGIN OF THE SEGMENTAL EXCRETORY ORGANS. 
Of these three clues to the past history of the spinal region, the 
segmentation manifested by the presence of vertebre is the least 
important, for in Ammoccetes there is no sign of vertebre, and their 
indications only appear at transformation. Especially interesting is 
the segmentation due to the excretory organs, for the evidence dis- 
tinctly shows that such excretory organs have steadily shifted more 
and more posteriorly during the evolution of the vertebrate. 
In Limulus the excretory organs are in the prosomatic region— 
the coxal glands; these become in the vertebrate the pituitary body. 
In Amphioxus the excretory organs are in the mesosomatic region, 
segmentally arranged with the gills. 
In vertebrates the excretory organs are in the metasomatic region 
posterior to the gills, and are segmentally arranged in this region. 
Their investigation has demonstrated the existence of three distinct 
stages in these organs: 1. A series of segmental excretory organs in 
segments immediately following the branchial segments. This is 
the oldest of the three sets, and to these organs the name of the pro- 
nephros is given. 2. A second series which extends more posteriorly 
than the first, overlaps them to an extent which is not yet settled, 
and takes their place; to them is given the name of the mesone- 
phros. 3, A third series continuous with the mesonephric is situated 
in segments still more posterior, supplants the mesonephros and 
forms the kidneys of all the higher vertebrates. This forms the 
metanephros. 
These three sets of excretory organs are not exactly alike in their 
origin, in that the pronephric tubules are formed from a different 
portion of the cclomic walls to that from which the meso- and 
