424 HOLMES. [Vol. XVI. 



head kidneys, and the important theoretic bearing of the sub- 

 ject, I was led to attempt to verify, if possible, Rabl's descrip- 

 tion of the development of these organs. I have not followed 

 the subject in detail, but have carried it far enough to satisfy 

 myself of the essential correctness of Rabl's results. The large 

 cells, ^i, v^, of Rabl are very conspicuous elements of the meso- 

 blastic bands, especially in later stages, as they increase consid- 

 erably in size. They may frequently be seen perforated by a 

 canal ; and one case was found in which the canal did not 

 extend entirely through the cell, but was narrowed to a point 

 near the posterior side. The shape of the canal would seem to 

 indicate that it arose by a sort of invagination at the anterior 

 end of the cell. It may be, however, that the perforation arises, 

 as has been shown in other cases, by the coalescence of a series 

 of intracellular vacuoles. The mesodermic origin of the giant 

 cell of the head kidney is a matter about which, I believe, there 

 cannot be any doubt. Whether a portion of the external canal 

 is ectodermic in origin, as Erlanger ('92) found in Bythinia, is 

 uncertain. The principal portion of the structure, however, 

 undoubtedly arises from the mesoderm. 



There can be little doubt, when we study the structure and 

 development of the larval kidneys of the pulmonate gasteropods, 

 that these organs represent true nephridia. As the definitive 

 renal organs of the gasteropods are regarded as nephridia also, 

 there occur in the pulmonates two pairs of these organs. Lar- 

 val kidneys similar to those of the pulmonates have been 

 observed in the embryos of Oncidium and some lamelli- 

 branchs. There are sac-like mesodermic larval kidneys in 

 Paludina and Bythinia, which may be the homologues of the 

 above structures. Whether or not the two pairs of renal 

 organs in Nautilus represent nephridial structures, it is quite 

 probable that the existence of the two pairs of nephridia should 

 be regarded as typical for the Mollusca. Whether this indi- 

 cates that the molluscan body is composed of two segments, 

 is a question which need not here be discussed. 



