402 THE ORIGIN OF VERTEBRATES 



each side of these mesohlastic plates. The subsequent downward 

 growth is brought about by the cells proliferating along the free 

 ventral edge of the mesoblast, these cells then growing ventralwards, 

 pushing their way between the yoke-cells and epiblast."- 



The derivation of the vertebrate pronephric segmental organs 

 from the metasomatic coxal glands of a primitive arthropod would 

 mean, if the segmental organs of Peripatus be taken as the type, 

 that such glands opened to the exterior on every segment, either 

 at the base of the appendage or on the appendage itself. It is 

 taken for granted by most observers that the pronephric segmental 

 organs once opened to the exterior on each segment, and then, 

 from some cause or other, ceased to do so, and the separate ducts, 

 by a process of fusion, came to form a single segmental duct, which 

 opened into the cloaca. Many observers have been led to the con- 

 clusion that the pronephric duct is epiblastic in origin, although 

 from its position in the adult, it appears far removed from all 

 epiblastic formations. However, at no time in the developmental 

 history is there any clear evidence of actual fusion of any part of the 

 pronephric organ with the epidermis, and the latest observer, Brauer, 

 is strongly of opinion that there is never sufficiently close contact 

 with the epidermis to warrant the statement that the epiblastic cells 

 take part in the formation of the duct. All that can be said is, that 

 the formation of the duct takes place at a time when the pronephric 

 diverticulum is in close propinquity to the epidermis, before the 

 ventral downgrowth of the myotome has taken place. 



The formation of the anterior portion of the pronephric duct is, 

 according to Maas in Myxine, and Wheeler in Petromyzon, undoubtedly 

 brought about by the fusion of a number of pronephric tubules, which, 

 according to Maas, are clearly seen in the youngest specimens as 

 separate segmental tubes; each of these tubules is supplied by a 

 capillary network from a segmental branch of the aorta, as in the 

 tubules of Amphioxus according to Boveri, and does not possess a 

 glomerulus. 



The posterior part of the duct into which the mesonephric tubules 

 enter possesses also a capillary network, which Maas considers to 

 represent the original capillary network of a series of pronephric 

 tubules, the only remnant of which is the duct into which the 

 mesonephric tubules open. He therefore argues that the pronephric 

 duct indicates a series of pronephric tubules, which originally extended 



