DEVELOPMENT OP PETROMYZON FLUVIATILIS. 825 



On Some Points in the Development of 

 Petromyzon fluviatilis.^ 



By 



Artbur E. Sbipley, B.A., 



Christ's College, Cambridge, Demonstrator of Comparative Anatomy in the 



University. 



With Plates XXVI, XXVII, XXVIII, and XXIX. 



The development of the Lamprey lias occupied the attention 

 of many embryologists during the last fifty years. Of these we 

 owe the most complete accounts of the changes through which 

 the egg passes to Max Schultze, Owsjannikow, Calberla, 

 Scott, Balfour, and Dohrn. I have recently worked through 

 the development of Petromyzon again, and worked out the 

 origin of several organs which have hitherto been incompletely 

 known. In many of the most important points my researches 

 confirm those of the earlier observers, and to these I have only 

 referred at such length as would make the account intelli- 

 gible ; in others, such as the persistence of the blastopore, the 

 origin of the ventral mesoblast, &c., I differ from previous 

 descriptions ; and some points, such as the development of the 

 heart, of the parts of the brain and cranial nerves, are worked 

 out for the first time. 



The material for this article was obtained by artificially 



* The differences between Petromyzon planeri and fluviatilis are so 

 slight, and the intermediate forms so common, that I am disposed to follow 

 Anton Schneider, and to consider them as varieties of the same species. This 

 species may conveniently retain the name fluviatilis, as opposed to the 

 larger form Petromyzon marinus. 



VOL. XXVII, PART 3 — Nb;W SEll. A A 



