No. 2.] FORMATION OF THE FISH EMBRYO. 449 



ist zuerst ein solider im Durchschnitte keilformiger Strang 

 der aus dem mittleren Theile des Sinnesblattes herauswachst. 

 Wiirde dieser Theil des Sinnesblattes gespaltet oder trifft der 

 Medullarstrang im Anfange seiner Entwickelung auf ein Hin- 

 derniss, so wird auf eine Strecke ein von unten her gespaltener 

 Medullarstrang enstehen miissen. . . , Ich habe es hiermit 

 zugleich auch ausgesprochen, dass nicht bloss eine Laesio con- 

 tinui in einem Keimblatte, sondern auch ein auf dasselbe 

 wirkender Druck oder Widerstand eine Duplicitat des aus 

 demselben sich entwickelnden Organes bedingen kann." 



If, then, elongation is due to an axial extension posteriorly, 

 he said, and the splitting of the embryo into right and left 

 halves is caused by division of its growing point, we have a 

 better explanation (than on the alternative hypothesis) of the 

 observed fact that very often there is an inequality in the 

 amount of material in the two divided parts. 



Professor Whitman's (17) masterly description, in 1878, of 

 the process of concrescence in Clepsine has furnished the 

 strongest support of the possibility of concrescence that has 

 been published. He demonstrated the presence of concrescence 

 for the leech, and it is not surprising that, with this definitely 

 established case before them, embryologists have extended the 

 same conclusions to other groups. Whitman says : " The germ- 

 bands in Clepsine, their epibolic growth and final conjunction 

 at the median neural line are so remarkably similar to the 

 embryonic rim and the process of neuralation in vertebrates as 

 to indicate a fundamental relationship. This similarity has 

 already been noticed by Semper and Hatschek, and adduced 

 as an argument in favor of a genealogical relationship between 

 the vertebrates and invertebrates. Of the justice of the com- 

 parison I am thoroughly convinced, and I propose here to add 

 some considerations in its favor which have, until now, passed 

 unnoticed." Whitman's work stands as the best ascertained 

 case of concrescence by apposition ; but, from the facts stated 

 in the preceding pages, I do not believe that we are now justi- 

 fied in extending exactly the same explanation to the develop- 

 ment of the fish. Rauber (12), in 1880, commits himself to 

 these statements in regard to the process of formation of the 



