1907.] NATURAL SCIENCES OF I'lII LADKLPIII A. 547 



form part of the alimentary canal arise early from these three cells of 

 the fourth quartet. In Planoccra apparently most of the protoplasmic 

 material has been separated from the cells 4a, 4b, and 4c in the previous 

 divisions, and they now contain little more than a mass of yolk granules. 



Lang, no doubt, overlooked the internal budding of the posterior cell, 

 4d, if such occurs in Discococlis. This internal division of 4d in Plano- 

 cera is very evident and striking. In other particulars the cleavage of 

 Planocera inquilina is so similar to that of other polyclads that it is 

 difficult to believe that this species differs so fundamentally in respect 

 to 4d. As stated before, practically all the previous work on polyclad 

 embryology has been clone on the living eggs alone. Such a division 

 as the internal budding of 4d might easily be overlooked in the living 

 opaque eggs. On this basis we might well conclude that in all prob- 

 ability such a division was overlooked by Lang and his predecessors. 



Wilson, however, who imdoubtedly was on the lookout for just 

 such a division, did not find it in Leptoplana. Wilson says that 

 he does not attempt to "describe the cleavage of Leptoplana in 

 detail, but only indicate its leading features." I cannot but 

 believe that in this statement is contained the reason why Wilson 

 did not find mesoderm arising from 4d. A process so evident and 

 significant as these divisions of 4d in Planocera can scarcely be con- 

 ceived of as a coenogenetic character of one or even a few species of 

 polyclads. In annelids and mollusks the bilateral division of 4c? and the 

 origin of mesoderm bands from these cells is without exception in the 

 numerous species so far studied. Both Wilson and myself have now 

 shown that the ectoderm of polyclads is segregated in the first three 

 quartets of micromeres, and that the second quartet gives rise to some 

 mesoderm. This process, so exactly paralleled in mollusks and annelids, 

 can now scarcely be doubted as constant in its main features for all 

 polyclads. Whether such a uniformity in the origin of the mesoderm 

 from 4d will be found to hold throughout these Turbellaria can onl}^ be 

 proven by further investigations on other species. I believe that 

 certainly the weight of evidence is in favor of this uniformity. 



The resemblance in the behavior of 4d in the polyclads to the homo- 

 logous cell in annelids and mollusks becomes only more striking as we 

 consider the details of its divisions. It is true that in annelids and 

 mollusks 4d divides into two bilateral halves before it buds cells into 

 the interior, while in Planocera 4d first buds a single cell into the seg- 

 mentation cavity and then each of the two di\ddes bilaterally (figs. 

 19, 25, 26). In either case exactly the same result is reached, and the 

 delay of the bilateral cleavage for one-cell generation can certainly be 

 very easily accounted for as a coenogenetic modification. 



