The development of the urinogenital organs of tlie lamprey. 59 



— metameric structure, convoluted tubules, typical nephrostomes and 

 glomeruli. 



A consideration of these cases certainly inclines one to be eclectic 

 and not to regard everything as primitive and typical because it oc- 

 curs in the form one happens to be studying. I shall, therefore, start 

 from the condition of the Selachian mesonephros as embodying the 

 essential characters of that renal organ in thp Craniota. The meso- 

 nephros of the Myxinoid may be more primitive in certain respects, 

 but the mode of origin of the tubules and the absence of nephro- 

 stomes opening into the body cavity seem to be secondary characters. 

 The admirable researches of Rückekt, Van Wijhe and Rabl have 

 elucidated the origin of the mesonephric tubules in sharks. Rückert 

 shows how they arise from the middle plate, or intermediate layer, 

 that piece of the somatic and splanchnic mesoderm connecting the 

 muscle and cutis plates of the myotome on the one hand with the 

 unsegmented somatic and splanchnic layers of the parietal mesoderm 

 on the other. This middle plate is from the first a tubular structure, 

 its lumen being a part of the coelom connecting the myocœle with 

 the splanchnocœle. It lies dorsal to the portion of the somatic layer 

 which gives rise to the pronephros and its duct, and ends blindly as 

 soon as the myotome separates from it. Later this blind end bends 

 over and opens into the pronephric duct, and the mesonephric tubule 

 has only to elongate, become contorted and acquire a glomerulus to 

 complete its development. Rabl (1896, p. 705—706 et al.) has shown 

 that but little of the splanchnic wall of the middle plate enters into 

 the formation of the mesonephric tubule, because it is mainly con- 

 sumed in producing the sclerotome. This throws the burden of 

 forming the mesonephric tubules largely on the somatopleure, which 

 is alone concerned in forming the pronephros and its duct. 



That there is no insuperable difficulty in deriving the mesonephros 

 of Amniota and Amphibia like Iclithyophis from embryonic conditions 

 similar to those of the sharks has been shown by Rückert and more 

 recently by Rabl. In fact, Sedgavick (1880) described the meso- 

 nephros of the chick and the Selachian as arising from the segmental 

 intermediate cell-masses uniting the myomeres with the unsegmented 

 parietal mesoderm. But the mesonephros of many Ichthyopsida is not 

 so readily reduced to the type of mesonephros formation seen in the 

 Selachians. At least three other modes of development have been 

 described. 1) It is claimed that the mesonephric tubules in certain 

 forms are differentiated from the retroperitoneal mesenchyma. 2) In 



