272 BULLETIN OF THE 



clear, and also suffers from misleading typographical and grammatical 

 errors ; but it is certain that the structure he describes lies within the 

 splanchnic peritoneum, and is not to be confounded, as "was done by van 

 Wijhe, with the partition between two pronephric tubes. Riickert says 

 ('88, p. 239), " Es [ein Paul Mayer'sches Quergefass] zieht dicht an der 

 medialen Grenze der Vornierenanlage vorbei und gelangt, indem es die 

 Leibeshbhle durchbricht, d. h. ihre Wandung vor sich herstiilpt, an die 

 Aussenfliiche des Darmes, wo es zwischen Ectoderm [soil wohl Entoderm 

 heissen] und Splanchnopleura gelegen, mit der rechten Subintestinalvene 

 confluirt." I cannot admit that the structure described by van Wijhe 

 is the homologue of the Amphibian glomus, nor do I believe that it 

 corresponds to the structure observed by Riickert. 



The mode of development of the excretory system is much alike in the 

 three groups of Amniotes. It seems, however, best in the present in- 

 stance to deal with the Reptiles separately from Birds and Mammals. 

 The most important of the works on the Reptilian excretory system is 

 perhaps the monograph of Braun ('77), which, however, is of little ser- 

 vice in elucidating the earliest stages. Weldon ('83) first gave a satis- 

 factory account of the early development. According to this author, the 

 first» trace of the excretory system in Lacerta is found in the region of 

 the intermediate cell mass, and consists of a series of vesicles (Segmen- 

 talblaschen of Braun), which have a strictly metameric arrangement. 

 Throughout a region of five protovertebra; (from the 8th to the 12th), 

 there appears on the external wall of these segmental vesicles a rod of 

 cells at first composed of discontinuous parts. This rod is the fundament 

 of the segmental duct ; in the region between two successive protover- 

 tebrse, it is budded off from the unmodified " middle plate" (Waldeyer), 

 or intermediate cell mass. Behind the twelfth protovertebra, the duct 

 grows backward, free from adjacent tissue. The rod of cells soon ac- 

 quires a lumen, continuous anteriorly with the cavities of the segmental 

 vesicles. 



The observations of Mihalkovics ('85) upon Lacerta agilis differ from 

 those of Weldon mainly in two particulars. In the first place, according 

 to Mihalkovics (pp. 42, 43), the most anterior three or four pairs of seg- 

 mental vesicles at the time of their origin communicate both with the 

 body cavity and with the protovertebral cavity. In other words, they 

 are formed as expansions of what I have termed the communicating 

 canal, or Mittelplattenspalten of the German authors. Some somites 

 in the series, however, may be without vesicles. Secondly, Mihalko- 

 vics (p. 48) maintains that the segmental duct buds off from the middle 



