W. G. VaalSfaiHe — EinbryoIo<jij of Eastyh)chus. 295 



in Fig. 34 ; tlie posterior part of the body is, however, much more * 

 prominently developed than in the earlier stage, and now forms 

 a distinct posterior median lobe. The large yolk-cells have by 

 this time become divided up, and most of the yolk has evidently 

 been absorbed, but some dark colored substance is left which may 

 be a remnant of it. 



The invagination between the posterior ventral lobes has also 

 become wider and more conspicuous, but I have not been able to 

 find at this stage any internal cavity with which it might communi- 

 cate. I have never seen one of the embryos eat anything, neither 

 will they ingest powdered carmine. 



Their bulk has not changed appreciably from that of the egg, and 

 so far they have been living on the yolk stored up within the cells. 

 When this is gone (as in the stage shown in Fig. 35) the embryo 

 soon dies. I have never been able to raise them any farther. If 

 they live longer they do not develop, but degenerate and lose their 

 activity. I found no indications of the peculiar chrysalis stage 

 which Girard described, and am convinced that he mistook some 

 other organism present with the embryos for a stage of the latter. 

 As might be expected, the development of JEustylochus resembles 

 closely that of the related polyclad Stylochus piUdium, described 

 by Goette (9, 10, II) and Lang (18). 



The later development of Planocera nebulosa does not differ much 

 from that of Eiistylochvs ellipticus. In the early cleavage stages 

 there is a little more difference in the size of the blastomeres. The 

 embryos are of course a little larger and the development is some- 

 what slower. I have kept them for nine or ten days before the eyes 

 began to appear. The left eye develops first, as in Eustylochus. 



In conclusion I wish to express my gratitude to Dr. Wesley R. • 

 Coe of the Sheffield Biological Ijaboratory, not only for the material 

 for which I am indebted to him, but for constant advice and assist- 

 ance during the course of the work, and especially for the personal 

 interest he has taken in it. My thanks are also due to Prof. A. E. 

 Verrill for valuable advice in regard to the illustrations and to Prof. 

 Sidney I. Smith, in whose laboratory the work was done. 



Sheffield Biological Laboratory of Yale University, June, 1899. 



