548 PROCEEDINGS OF THE ACADEMY OF [DeC. 



Conklin (97) was the first to point out that in the gasteropod Crepi- 

 dulas the cell 4d gave rise to both endoclerm and mesoderm. In Crepi- 

 dula four approximately equal cells are at first formed from M. The 

 two lower and external are pure entoblasts. Each of the two upper 

 cells later gives off another entoblastic cell before they give rise to the 

 mesoblast bands. Since that time numerous observers have found that 

 in both annelids and mollusks the cell 4d is mesentoblastic. Wilson 

 (98), in the paper so often referred to above, shows that a reinvestigation 

 of Nereis proves that a number of small eutoblast cells are budded off 

 from the two halves of 4d l^efore these form the mesodermal l)ands. 

 He also shows that a series of stages may be found in different annelids 

 and mollusks, ranging from a single pair of minute "vestigial " entoblast 

 cells in Aricia and Spio to Nereis where from six to ten small cells are 

 budded off, and to the condition in Crepidula where more than half the 

 bulk of 4d forms endoderm. Around these and other facts Wilson has 

 elaborated a beautiful theor}^ of ancestral reminiscence. To this series, 

 agreeing very closely with Crepidula, we may add the polyclad Plano- 

 cera inquilina. Here, as in Crepidula, the two lower superficial cells 

 derived from 4d are purely entoblastic and, as has been shown, give rise 

 to very nearly all of the alimentary canal. Two more small cells, 4(F-^-^ 

 and 4d^-^V, derived from the two upper cells of 4r/, are probably added to 

 the endoderm, while the remainder forms mesoderm. This close, almost 

 astonishing, agreement of Planocera with annelids and mollusks 

 cannot be without some significance. As Wilson (98) (p. 13) says: 

 "If we accept Lang's view, which is supported by a large amount of 

 evidence, that the platodes are not very far remo\'ed from the ancestral 

 prototype of annelids and mollusks, we should expect to find in the 

 polyclad a mode of cleavage to which that of the higher forms can in its 

 main features be red\iced." 



Before Wilson's work this resemblance between polyclads and higher 

 forms had seemed to be "only in the jorm of cleavage and not, so to 

 speak, in its substance." I believe that now this difficulty has been 

 entirely removed, and the polyclad cleavage not onl}^ conforms to the 

 higher types in its "main features" but, as we have seen, in many of 

 its details. These facts here set forth cannot l;)ut lend additional 

 W'eight to the view already expressed on comparative anatomical 

 grounds that the polyclads represent an offshoot from the same 

 ancestral branch which later gave rise to the annelids and mollusks. 

 On the other hand, it is a remarkable and interesting fact that phyla 

 which must have separated from the common stock and from each 

 other long ages ago still show such remarkable resemblances in their 



