516 PROCEEDINGS OF THE ACADEMY OF [DeC^ 



the third. Wilson's work was not a detailed study of the develop- 

 ment, and it lea\-es the impression that there may be something still 

 undiscovered in the embryology of these interesting worms. Most of 

 Wilson's work and all of Lang's was on the living egg. P]very student 

 of embryology knows how luisatisfactory this method is for the late 

 stages of segmentation, imless checked by properly fixed and stained 

 material. 



In view of these facts it has seemed worth while to enter into a more 

 or less detailed account of the cell lineage of this form and gain, if 

 possible, data to support or refute the theory of cellular homology. 

 Besides, such data should throw some light on the phylogeny of this 

 class of the Turbellaria. 



Lang has given so complete a review of the literature on polyclad 

 embryology previous to 1884 that it would be mere repetition to go 

 over that in detail here. The earliest investigators were Girard (1846- 

 1 854) , Vailliant ( 1 866-1 868) and Kef erstein ( 1 868) . Of these the work 

 of Keferstein was by far the best. Three other investigators had 

 studied polyclad development previous to 1884. The first of these 

 was Hallez (1878-1879). Hallez recognized but one quartet of micro- 

 meres. From this the ectoderm arose. The macromeres budded off 

 four small cells at the oral pole which he believed formed the mesoderm. ' 

 He described four other later buds at the oral po\e which formed the 

 wall of the gut. The work of Goette (1878-1882) and Selenka (1881) 

 followed close on that of Hallez. Goette also observed but one quartet 

 of micromeres in Stylochus pillidium. When the ectoderm had 

 reached the equator of the egg two to four small cells were formed at 

 the oral pole (lower endoderm). Later the large macromeres l^uddeil 

 large cells towards the aboral pole. These cells formed the upper 

 endoderm. From the upper and lower endoderm the wall of the 

 alimentary canal was formed, while the lai-ge macromeres 'became 

 food yolk. Goette foiuid no mesoderm. 



Selenka (1881) determined that two quai-tets were given off. Ac- 

 cording to him the first formed ectoderm and the second the mesoderm. 

 Four small cells were formed at the oral pole (lower endoderm of 

 Goette), from which he believed the entire wall of the alimentary canal 

 was formed. He found no upper endoderm. 



Lang (1884) found that three quartets of micromeres were formed. 

 As already stated he believed the first formed the ectoderm, the second 

 and third the mesoderm. From the large macromeres four small cells 

 were formed at the oral pole (lower endoderm). Then each of the large 

 macromeres (fourth quartet) budded towards the aboral pole an upper 



