32 BIOLOGICAL LECTURES. 



from d 2 and <r 2 ), though I could not positively determine which. 

 This was immediately followed by Treadwell's discovery that 

 in the annelid Podarke mesoblast cells are formed from three 

 cells of the third quartet, namely, the anterior median and the 

 two lateral cells (# 3 , 3 , <^ 3 ). It was thus shown that in at least 

 four genera of mollusks and two of annelids a part of the meso- 

 blast has an origin which recalls that of the polyclades, and the 

 view is irresistibly suggested that the formation of this ecto- 

 mesoblast in one, two, or three quadrants in the higher types 

 is a vestigial process or ancestral reminiscence of what occurred 

 in all four quadrants in the ancestral prototype and still persists 

 in the polyclade. 



The second difficulty i.e., the origin of the ectoblast has 

 entirely disappeared upon a reexamination of the cell-lineage of 

 a polyclade (Leptoplana) which I was enabled to make in the 

 summer of 1897. In this form careful study shows in the 

 clearest manner that the formation of ectoblast is not confined 

 to the first quartet, but that all of the twelve cells of the first 

 three quartets contribute to the ectoblast, precisely as is the 

 case in the annelids and mollusks (Fig. 3, A; Fig. 4, for details). 

 A comparison with Lang's figures gives every reason to believe 

 that the same is true in Discocoelis and the other forms studied 

 by him, and that on this point he fell into an error which was 

 certainly very pardonable at the time. The quartet-cells from 

 which in the polyclade the mesoblast arises are, therefore, not 

 pure mesoblasts, as Lang supposed, but are mesectoblasts, pre- 

 cisely like the cells from which the " larval mesoblast " arises 

 in Crepidula or Unio. 1 



The researches reviewed up to this point have cleared up the 

 contradiction relating to the second quartet. Passing now to 

 the third and fourth quartets, we find that the newer re- 

 searches have introduced a new difficulty with respect to each 

 of these quartets ; but the new difficulties differ from the old in 



1 In Leptoplana each cell of the second quartet divides off in succession three 

 ectoblast cells before the delamination of mesoblast into the interior occurs at the 

 fourth division (Fig. 4). In Unio, according to Lillie, the larval mesoblast is 

 definitely separated at the third division of the micromere (a 2 ). Professor Conklin 

 informs me that in Crepidula the ectomesoblast is formed at about the fourth or 

 fifth division of the micromeres (a 2 , b 2 , c 2 }. 



