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 — i.e. 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 vertebrae is the least 

 important, for in Ammoccetes there is no sign of vertebras, 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- 

 pliros. 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 

 metampliros. 



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 ccelomic walls to that from which the meso- and 



