596 MONTGOMERY— MORPHOLOGY OF THE [April 24, 



segmental arrangement, in their function, and in their histological 

 structure, the excretory organs of Amphioxus and the nephridia of 

 Phyllodocc are in all essentials identical." In a second communica- 

 tion Boveri ( 1904) maintained the occurrence of true nephrostomes, 

 and held the solenocytes to be modified peritoneal cells and not to be 

 covered by a peritoneal investment. 



Unfortunately nothing is known of the development of these 

 structures. 



36. Vertebrata (Craniota). 



With regard to the excretory organs of this group I shall deal 

 reather summarily, because they have been much more studied than 

 the excretory organs of other animals, and because most of the larger 

 contributions on the subject deal extensively w^ith the literature. 



Nephridia. — Good reviews of the embryogeny of these structures 

 have been presented particularly by Riickert (1892), Boveri (1892), 

 Wheeler (1899) and Brauer (1902). There are three kidney sys- 

 tems which occur in the ontogeny in the order of their naming; the 

 pronephros, mesonephros and metanephros. The first two occur 

 in all vertebrates, the third in amniotes only. The pronephros is 

 purely an embryonic structure except in Bdellostoma, Lepidosteus 

 and some Teleosts {e. g., Fierasfer) in which it functions also in 

 the adult. The mesonephros is the adult kidney of all other anam- 

 niotes, and the metanephros of the amniotes. All these organs are 

 paired and segmented. 



Pronephros. — This develops in the anterior trunk segments as 

 serial solid thickenings of the somatic mesoblast, each of which 

 secondarily becomes tubular and pushes towards and opens into the 

 coelom. Their lateral ends tmite to form the collecting tubule. The 

 arterial connection is in most cases by a paired glomus, an unseg- 

 mented vascular inpushing of the dorsal peritoneum medial from and 

 opposite the nephrostomes. The duct, generally known as the seg- 

 mental duct, also as the pronephric or Wolffian duct, arises just 

 lateral from the tubules and grows back from them to open into the 

 cloaca ; in the Selachii and Mammals, possibly also in Lepidosteus, 

 it is ectoblastic and joints secondarily with the tubules; in all other 

 forms it arises from the somatic mesoblast in conjunction with the 



