NOTES. 



99 



cular fold of the so-called collar in Balanoglossus is of the same 

 nature as the atrium of Amphioxus ; but this is somewhat doubtful. 



5. (p. 70.) For an admirable critical and historical account 

 of our knowledge of the development of the excretory system in 

 the different groups of Vertebrates, the reader may be referred to 

 the report on the " Entwickelung der Excretionsorgane" by Pro- 

 fessor RUCKERT, in Merkel and Bonnet, Ergebnisse der Anatomic 

 uiiii Entwickelirngsgeschichte, Band I., 1891. It will be sufficient 

 to note here that the ectodermic origin of the pronephric duct, 

 as briefly described in the text, only holds for the Selachians and 

 Mammals. It was first discovered in the latter by GRAF SPEE in 

 1884, and confirmed later by F LEMMING. In the former it was 

 discovered independently by VAN WIJHE and RUCKERT (1886-8). 

 On the contrary, in Petromyzon, Amphibia, Reptiles, and Birds, the 

 duct does not arise from the ectoderm. 



Van Wijhe denied the segmental fusions with the ectoderm of 

 the pronephric tubules in Selachians as described by Riickert. 

 The account given by the latter author has, however, been 

 indirectly confirmed by the observations of FELIX on the chick, 

 where the pronephric outgrowths were found in some cases to 

 undergo a transitory fusion with the ectoderm. 



BOVERI has attempted to show how the origin of the pronephric 

 duct can be imagined to have been gradually transferred from the 

 ectoderm to the mesoderm. Finally, it may be noted that, 

 whereas RUCKERT compared the pronephric tubules with the 

 Annelid nephridia, SEMPER and others employed the mesonephric 

 tubules for the comparison. The fallacy of the latter comparison 

 was first pointed out by FURBRINGER. 



6. (p. 74.) In 1887 PAUL MAYER discovered that the sub- 

 intestinal vein in the Selachian (Pristiurus) embryo communicated 

 with the dorsal aorta, by a series of six segmental vessels which 

 passed up around the intestine on the right side only. Correspond- 

 ing to them on the left side he found short, blind outgrowths from 

 the dorsal aorta similar to those figured in the text in connexion 

 with the pronephros of Ichthyophis. Paul Mayer's connecting 

 vessels soon become aborted with the exception of one which 

 enlarges and forms the proximal portion of the umbilical artery. 

 In the following year it was shown in a brilliant manner by 



