596 MORGAN AND HAZEN. [Vol. XVI. 



the mesoderm has a double origin — the duality, however, being 

 a matter of definition. In Amphioxus there is nothing indi- 

 cating that the mesoderm is derived from more than a single 

 source. 



In Ciona the blastopore closes on the dorsal side, and the 

 longitudinal axis of the embryo seems to be at right angles to 

 the primary or gastrula axis. This would be the case if the 

 dorsal lip grew posteriorly, without the embryo changing 

 shape. On the other hand, there are facts in the development 

 that make it possible to interpret this backward growth of the 

 dorsal lip as the result of a change in the shape of the entire 

 embryo. The gastrula axis, in such a case, would shift during 

 the closure of the blastopore. The shifting would take place 

 in such a way that the dorsal side of the axis is carried back- 

 ward and the ventral forward. The result would be that the 

 anterior end of Ciona would agree with the anterior end of 

 Amphioxus, We offer this only as a suggestion, for by this 

 means the orientation of Amphioxus and of Ciona would be 

 made to agree. 



Bryn Mawr College, Bryn Mawr, Pa., 

 May 29, 1898. 



