68 EDWARD MCCRADY, JR. 



the duct and the walls of the tubules are continuous from the 

 very beginning. 



The important features of this development from a theo- 

 retical point of view are: that pronephric tubules, meso- 

 nephric tubules, and wolffian duct all arise from the same 

 anlage; that this anlage is a longitudinal unsegmented struc- 

 ture which is continuous from its anterior end back to the 

 region of the prospective cloaca ; and that this anlage has for 

 awhile the form of a ridge on the coelomic lining. 



All of these features are in accord with Burlend's theory, 

 which is, in part, that "the nephric system originated phylo- 

 genetically from a right and left 'primitive nephric groove' 

 which became differentiated from the somatopleuric meso- 

 derm lining the primitive splanchnocoel ; " that "the groove 

 became closed off from the splanchnocoel at intervals, and 

 thus a duct .... was evolved." This is to be contrasted 

 with the familiar textbook account, which may be called the 

 Brauer-Rabl theory, according to which the tubules arise first 

 as separate segmental units. Their distal tips then grow to- 

 gether to form a duct (Sammelgang) which from the pos- 

 terior end of the pronephric region sprouts independently 

 (Endabschnitt) all the way back to the cloaca. 



There is much evidence that the Brauer-Rabl theory is 

 based upon some special cases (principally that of Hypo- 

 geophis) in which the intermediate mesoderm becomes seg- 

 mented before nephrogenesis begins. Burlend was able to 

 adduce strong contradictory evidence from cyclostomes, 

 elasmobranchs, teleosts, ganoids, and dipnoi, all of which 

 groups are more primitive than Hypogeophis and should ac- 

 cordingly be more significant. The pronephros in all the 

 amniotes is so vestigial that not much hope has been enter- 

 tained of securing interesting evidence from this source. 

 The opossum evidence, supporting as it does the Burlend 

 theory, is of particular interest, therefore, because it comes 

 from the highest group of amniotes the mammals. 



One implication of the theory which is of general interest 

 should be mentioned here. If the theory of the non-metameric 



