40 LILLIE. [Vol. X. 



to convince embryologists that all tissues lying between ecto- 

 derm and entoderm are not, of necessity, either mesothelial or 

 mesenchymal, in the Hertwigian sense, and that the occurrence 

 of other elements is not isolated. It does not follow, therefore, 

 that we must deny the homology of the mesoblast throughout. 

 On the contrary, the tendency of the work being done, both 

 on Vertebrata and Invertebrata, is to demonstrate that a por- 

 tion, at least, of what was previously called mesoblast, is 

 strictly homologous, both in origin and fate, within the limits 

 of the Vertebrata and Invertebrata, respectively. When, for 

 instance, we see that, in a widely varying series of Annelida 

 and Mollusca, the mesoblast is derived from a cell of identical 

 lineage, we must grant that a new and strong proof of homology 

 is adduced. When, on the other hand, we meet with a form 

 like Unio, where another conspicuous source of mesoblast is 

 found, we shall decide on a priori grounds that such a source 

 probably exists in other forms, but that it is comparatively 

 inconspicuous. 



Applying the test of observed facts to this decision but con- 

 firms its justice. Ziegler (No. 6^), for instance, says, in 

 speaking of the mesenchyme of Cyclas Cornea (p. 531), " Es 

 ist mir daher nicht unwahrscheinlich, dass an bestimmten 

 Stellen des Ectoderms Mesenchymzellen vom Ektoderm aus 

 entstehen." Here is another lamellibranch in which it is "not 

 improbable" that the ectoderm contributes to the mesoblast. 

 What is the fate of these "fraglichen Zellen " .? They form 

 the single muscle-cells (Strangzellen) which are so numerous 

 in the larvae of lamellibranchs. Now, as I shall show, the 

 same cells in Unio are derived from the larval mesoblast, also 

 the source of the larval adductor muscle, which has nothing 

 whatever to do with the adductors of the adult (F. Schmidt, 

 No. 31 ; Braun, Nos. 6 to 9); it is, in fact, nothing but a bunch 

 of Strangzellen associated for a common end. I shall speak of 

 this in more detail in the third part of my paper. Stauffacher, 

 the last author on Cyclas, seems to regard ectodermal partici- 

 pation in the formation of the mesenchyme as very probable. 

 Lankester (No. 54) is very positive about the derivation of 

 many of the "branching cells" within the segmentation 



