ORIGIN OF THE PRONEPHRIC DUCT IN 



SELACHIANS. 



EMILY RAY GREGORY. 



A RENEWED study of the origin of the pronephric duct in 

 Selachians was suggested to me by Dr. W. M. Wheeler of the 

 University of Chicago, in whose laboratory the work has been 

 carried on, and to whom I am indebted for the material used 

 and for helpful directions. 



There have been different opinions on this subject, which are 

 well represented by the papers of Joh. Ruckert, 1 J. W. van 

 Wyhe, 2 and Carl Rabl, 3 to which reference will be made later. 

 The paper by Laguesse, 4 who also used Acanthias, was inac- 

 cessible. The material used was AcantJiias embryos, of which 

 there were quite a number, showing different stages of develop- 

 ment. Several methods of killing and hardening were em- 

 ployed, but all gave essentially the same results. Cross sections 

 were examined with great care, so that no misinterpretation 

 might occur through slanting sections; and where a question of 

 fusion was involved, a Zeiss J^ immersion lens was employed. 



In an embryo of 25 somites the Anlage of the pronephros is 

 seen to consist of segmental outgrowths of the somatic layer 

 of the somites, beginning at the seventh and extending over six 

 segments. There is a slight difference in the development on 

 the two sides of the embryo in respect to the position of the 

 anterior end. These outgrowths are connected with the ecto- 

 derm at their outer edge, and in places this layer seems to 



1 Ueber die Entstehung der Excretionsorgane bei Selachiern, Joh. Ruckert, 

 Archiv.fiir Anat. tind Phys., 1888. 



2 Ueber die Mesodermsegmente des Rumpfes und die Entwickelung des 

 Exkretionssystems bei Selachiern, J. W. van Wyhe, Arch. f. m. Anat., Bd. 33, 

 1889. 



3 Ueber die Entwicklung der Urogenitalsystems der Selachier, Carl Rabl, 

 Morph.Jahrbiich, Heft 4, 1896. 



4 Sur le developpement du Mesenchyme et du pronephros chez les Selaciens 

 (Acanthias), Comptes rendus kebdomadaires des seances et memoires de la Societe de 

 Biologie, tome iii, serie ix, 1891. 



