No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 423 



is entirely of ectodermal origin. In a paper on the development 

 of the fresh-water pulmonates, Rabl ('75) described this organ, 

 which he mistook for a part of the nervous system, as arising 

 in a somewhat similar way to that described by Fol. 



According to Wolfson's account of the larval kidney in 

 Lymnaea, a large velar cell on either side of the embryo is 

 pushed inward, becomes perforated by a canal, and gives rise 

 to a hollow outgrowth which is directed anteriorly and opens 

 by a ciliated mouth. Wolfson holds, in opposition to Fol, that 

 the Urniere are unicellular organs, basing his opinion on the 

 study of a large number of sections. 



The account of the origin of the larval kidney given in Rabl's 

 later paper, " Ueber die Entwicklung der Tellerschnecke " ('79), 

 differs radically from the foregoing descriptions. According to 

 Rabl's later account, the origin of the larval kidneys can be 

 traced back to a large cell lying in the mesoblastic bands. 

 These large cells, according to Rabl, result from the first divi- 

 sion of the mesoblastic teloblasts, though it is more probable 

 they arose from the second (see p. 407). By the teloblastic 

 budding of the two middle cells, two rows of smaller cells are 

 produced which carry forward the large cells, v^, v^, at their 

 ends. These large cells also bud off small cells anteriorly, like 

 the teloblasts. Each mesoblastic band comes thus to consist 

 of a large cell with a row of smaller cells in front, followed by 

 another large cell with a similar row in front of it. The cells 

 i\ and v^ become perforated by a canal and come to be elongated 

 and bent, forming the giant cell of the larval kidney. Then 

 some of the cells lying in front and behind the giant cells also 

 become perforated and form the anterior and lower arms. The 

 larval kidneys, therefore, according to Rabl, are multicellular 

 organs and entirely mesodermic in origin. The teloblastic 

 budding of the protoblasts of the larval kidneys is a fact of 

 much interest. It recalls the behavior of the nephroblasts 

 described by Whitman ('78) in Clepsine, only in Planorbis the 

 two teloblastic series are placed end to end, instead of side by 

 side. Such a difference might easily be produced by a varia- 

 tion in the direction of one of the divisions of the teloblasts. 



In view of the contradictory accounts of the origin of the 



