NOTES ON THE DEVELOPMENT OF PETROMYZON. 



W. B. SCOTT, 

 College of New Jersey, Princeton. 



The present paper is a continuation of an article which ap- 

 peared in the " Morphologisches Jahrbuch " for i88i, and, hke 

 that article, is for the most part founded upon study of the great 

 series of sections prepared by the late Dr. Ernst Calberla, of 

 Heidelberg, which was placed in my hands by Professor Gegen- 

 baur. This paper has been nearly ready for publication for 

 several years past, but its completion has been long delayed by 

 circumstances which need not be enumerated here. 



Since the appearance of my first article several errors have 

 been detected in it, some by other observers and some by my- 

 self; but as these have all been commented upon by Shipley, it 

 is needless to dwell upon them further than to mention them. 

 Thus my account of the formation of the ventral mesoblast by 

 splitting from the yolk-hypoblast, of the development of the 

 pronephros from diverticula of the segmental duct, of the for- 

 mation of the anus and neurenteric canal, are incorrect. With 

 regard to the epithelial layer of the central nervous system, a 

 word of explanation may not be out of place. The existence 

 of this layer in the first rudiment of the nervous axis has been 

 affirmed by Calberla and myself, and denied by Balfour and 

 Shipley. At Balfour's request I carefully reexamined all of 

 Calberla's preparations which bear upon this point, and was, at 

 the time, fully convinced of the accuracy of Calberla's state- 

 ments. I have not had an opportunity of again examining the 

 question since the appearance of Shipley's paper; and, in view 

 of his positive statements, my confidence in my former account 

 is somewhat shaken, especially as sections prepared by the 

 more recent methods are so much clearer than those prepared 

 by the methods in use in Calberla's time. Nevertheless, the 

 evidence appeared so perfectly satisfactory, when I last had the 



