I'.X)?.] NATUltAI. SCIKXCKS OF IMH r.ADKLI'HI A. 549 



•early ontogenies. Vet I tliink lliis is the only intcrpretaticjn we can 

 put upon the facts. With our present knowledge of " Entwickelungs- 

 niechanilv" we cannot interpet these resemblances as due to external, 

 mechanical conditions. Why the ectoblast should be segregated in 

 three and only three quartets in polyclads, annelids, mollusks and, as 

 Biglow (02) has shown, in some crustacesc, and why in these same 

 grou{)s mesoderm should arise from the posterior or left posterior cell 

 of the fourth quartet and from no other of this quartet, are questions 

 wliich do not readily lend themselves to a mechanical explanation 

 under our present theories. We must for the present at least regard 

 these resemblances as facts of heredity and hence of phylogentic value. 



The origin of the alimentary canal in Planocera is unique among 

 animals, so far as I am aware. The whole of the alimentary canal arises 

 from a portion of the posterior cell of the fourth quartet, while the other 

 three cells of this quartet and all four of the macromeres are used as food 

 or degenerate and give rise to no morphological structure. Not one of 

 the last seven cells mentioned ever divides after its formation at the thirty- 

 two-cell stage. Surprising and unique as this phenomenon may be. it 

 does not necessarily invalidate our present conception of the develop- 

 ment of germ layers or their organs. Since the establishment of cell- 

 lineage work it has become well known that many embryonic cells are 

 formed in early stages which are destined never to divide again, nor to 

 take any further part in the organization of the embryo. Compare 

 for example the "turret" cells of certain mollusks as Crepidula {Conklin, 

 97). As has been pointed out, it is well known that in many annelids 

 and molluslcs the cell -id gives rise to a portion of the alimentary canal. 

 In these animals however the other three cells of this quartet as well 

 as the macromeres take part in the formation of the digestive tract. 

 Indeed these latter cells furnish the major portion of the alimentary 

 tissue. With these facts in mind, we must regard the condition of the 

 macromeres and the three anterior cells of the fourth quartet in Plano- 

 cera as reminiscent of a time when all eight of these cells took part in 

 the formation of the alimentary tract. Thus the embryology shows 

 that this worm is specialized in this respect and must long ago have 

 left the track which led on to annelids and mollusks. 



This peculiar development of the alimentary canal in the polyclads 

 offers the suggestion that it may be a step toward the development of 

 8uch forms as the acoelous rhabdocoeles, in which the alimentary canal 

 is altogether absent. But tiie embryology of these as well as other 

 turbellaria present such great variations from the type found in the 

 )iolyclads that it is useless to speculate along this line. 



