36 BIOLOGICAL LECTURES. 



with that of the Turbellaria was gradually replaced, though 

 still persisting in a reduced form as the larval mesenchyme. 



I would not at present urge the acceptance of this daring 

 hypothesis ; but in the light of later research it has become 

 highly significant, and whether true or false is of great interest 

 as giving a clear picture of how such a process of substitution 

 may have been possible. 



2. In the second line of evidence lies Lillie's discovery that 

 the ectomesoblast of Unio (derived from a 2 } gives rise to purely 

 larval transitory structures ; namely, to the adductor muscle 

 and the scattered contractile myocytes of the Glochidium larva. 

 In the annelids, too, the same conclusion seems probable, and 

 my friend Professor Treadwell informs me that in Podarke 

 there is every reason to believe that the ectomesoblast (derived 

 from a*, &, d*) is entirely devoted to the formation of the ring- 

 muscle and myocytes of the trochophore, which apparently take 

 but an insignificant part, if any, in the building of the adult body. 

 This result tallies with the view that the ectomesoblast forma- 

 tion in the higher types is a reminiscence of the ancestral process 

 still existing in the polyclade, but in the higher forms relegated 

 to the early stages, and even in them is more or less reduced. 1 



1 Eisig has very recently (Mitth. Zool. Station, Neapel, xiii, i, 2, 1898) published 

 the results of a study of the cell-lineage and later development of Capitella, which 

 are totally at variance with the view here suggested, and the facts on which it is 

 based. Broadly speaking, his results exactly reverse those of all the authors cited 

 above, the mesoblast-bands (" Coelomesoblast ") being derived from the third quartet 

 (c 3 - 1 and d' 3 - 1 ), while the larval mesoblast ("Paedomesoblast ") arises from a portion 

 of M (d*), the remaining portion giving rise to ectoblast. If well founded, this 

 result is not only fatal to the view I suggest, but is, I believe, nothing less than a 

 reductio ad absurdum of the whole cell-lineage program, regarded as a method of 

 morphological research. No one will lightly call in question the results of so 

 conscientious and eminent an observer ; and they must be regarded as by far the 

 most serious obstacle that the morphological study of cell-lineage has thus far 

 encountered. I will not attempt to explain away this adverse evidence, based on 

 so prolonged and thorough a research. Tt should not be forgotten, however, that, 

 as Professor Eisig is himself careful to point out, the nature of the material has 

 forced him to contend with great difficulties, since the eggs are normally distorted 

 by pressure (a factor which, as I have experimentally shown in Nereis, may greatly 

 alter the form of cleavage) between the membranes of the tube ; and, further, the 

 development cannot be continuously followed in life. A result, based on this 

 material, which stands in such flat contradiction to what is known in other and 

 more favorable forms, must await the test of further research. 



