( 843 ) 



l)y E. 1). Wii.soN (Ainials of Ihc New-York Acadoniv of Sciences, 

 vol. XI |). Jo). Fi'oni liis piiblicalion we may conclude as follows: 

 J. That a cell-coiiple as represciiled Iw LAN(i for Discocoelis, is 

 also present in Leploplana, which Mkad has compared to the mother- 

 cells of the mesohlastbands of Annelids and Molluscs, t(/th(>H(/h/ro)n 

 this cell-couple no mesobla.'it develops in either (jeims of Phithelmiuths. 



2. That, moreover, with Leptojdana, four cells of the second cell- 

 quartet (counted from above) become the mother-cells of "larval mesen- 

 cliym", that they remove to the interior and that bv further subdivision 

 they gradually furm'sh the whole mesoblast of Leploplana. This origin 

 of the mesoblast in Plathelminths was also already known to Lang. 



3. That also with Molluscs (Unio, Crepidula) and probably also 

 with certain Annelids (Aricia), beside the two symmetrical mesoblast 

 bands still another source of mesoblastic tissue occurs, which is 

 directly comparable to the source of larval mesenchym mentioned 

 in 2, and that also these mesenchym mother-cells originate from cells 

 l)elonging to the ecloblast quartet, as with the Plathelminths. 



4. That consequently it may be considered a settled fact that 

 with Annelids and Molluscs the mesoblast has a twofold origin ^). 



CoxKLix (Vol. XIII, Journal of Morphology, p. L5I) has emphasized 

 that thus mesoblast is furnished by each of the four quadrants, 

 viz. the mesenchym by the micromeres of the second cpiartet 

 of A, B, and C, the mesoderm-bands l)y ]). 



This latter phenomenon is always coniiected with lengthening of 

 the body and with teloblastic growth, even with animals like Lamel- 

 libranchia and Gastropoda, although the latter are Jiot generally 

 looked upon as longitudinally develo])ed forms. From this Conkt.ix 

 justly inferred that the radial mesoblast has a still more [»iimiti\e 

 character than the bilateral. 



Whoever considers more closely the corres{)ondence here noticed 

 in the development of the Polyclada with that of the Annelids and 

 Molluscs, will have to acknowledge that only that solution can be 

 satisfactory which considers the two cells, mentioned in J, as the last 

 remnant of the no longer fully de\elo})ing mesoblasl-bands with 

 the degeneration of which the disappearance of the coelom and of 

 a distinct metamerism has gone hand in hand. 



1) I may briefly call attention to the tact that I also pleaded lor a manyfold 

 origin of the mesoblast with mammals, on account of what had been found in 

 Tarsius (Verb. Kon. Ak. v. Wet. Amsterdam, vol. VIII n". 1002, p. G'.») and that 

 I concluded from it that the mesoderm is not eiiuivalent to llie two primary germ- 

 layers, but that Kleinenberg was right when he qualilieil it as a complex of 

 originally independently developing organs. 



