362 GEORGE A. BATES 



Figure 49 shows the caudal extremity of the pronephros in an 

 embryo of 4.5 mm., with the duct just beginning, and here, just 

 at the junction of the pronephros and duct, is a mitotic figure. Its 

 presence at this point and the early stage of development seem- 

 ingly can have but one meaning, namely, that the anlage is grow- 

 ing by the division of its own cells. This is not an isolated ex- 

 ample as may be seen by reference to the figures. 



All these examples, coupled with other evidence, seems to 

 point inevitably to the conclusion that, not only is the anlage of 

 the duct a direct outgrowth from the pronephros, and therefore 

 mesodermic in origin, but its subsequent growth is accomplished 

 by the division of its own cells and it in nowise receives contribu- 

 tion of cells from the ectoderm. 



Respecting the theoretical phase of the problem, it seems clear 

 that the relation of the anlage of the duct is secondary and that 

 the duct is not developed from an ectodermic groove. If the 

 nephridia of vertebrates ever opened to the surface through ecto- 

 dermic pores, and later into a canal arising from the ectoderm, 

 ontogeny has failed to repeat phylogeny in Acanthias, for there 

 is no evidence of such arrangement in the embryological develop- 

 ment of this form. 



BIBLIOGRAPHY 



Balfour, F. M. 1878 A monograph on the development of elasmobranch fishes. 



London. 

 Beard, J. 1887 The origin .of the segmental duct in elasmobranchs. Anat. 



Anz., Bd. 2. 

 Felix, W. 1890 Zur Entwicklungsgeschichte der Vorniere des Hiihnchens. 



Anat. Anz., Bd. 5. 

 Flemming, W. 1886 Die ectoblastische Anlage des Urogenitalsystems beim 



Kaninchen. Arch, f . Anat. und Phys. 

 Gregory, E.R. 1897 Originof the pronephric duct in selachians. Zool. Bull., 



Boston, vol. 1, pp. 123-129. 



1898 The pronephros in Testudinata. Science, N. S., vol. 7. 



1900 Observations on the development of the excretory system in 



turtles. Zool. Jahrb.^Anat. Abt., Bd. 13, pp. 683-714. 

 Haddox, a. C. 1887 Suggestion respecting the epiblastic origin of the segmen- 

 tal duct. Proc. of the Royal Dublin Society, vol. 5, February. 

 Harrison, R. G. 1903 Experimentelle Untersuchungen ueber die Entwicklung 



der Sinnesorgane der Seitenlinie bei den Amphibien. Arch. f. mikr. 



Anat., Bd. 63. 



