^6 MARINE BIOLOGICAL LABORATORY. 



veloped, individualized head, which carries within itself 

 a minute, rudimentary trunk — just as in a seed two 

 huge modified leaves, the cotyledons, carry between 

 them the minute germ of the stem, root, and foliage- 

 leaves. If the cells in question can be shown to be 

 always present the trochophore is not a diploblastic or- 

 ganism, but a triploblastic one, and Kleinenberg's com- 

 parison of the trochophore to a medusa falls to the 

 ground. Kleinenberg asserts that the trochophore con- 

 sists at first of the ectoblast and entoblast alone, the 

 mesoblast being a later formation. This conclusion is 

 based upon the study of Lopadorhynchus, in which the 

 mesoblast is apparently not present at the start but is 

 afterwards split off from the ventral ectoblast. This 

 result has always seemed a very puzzling one, which 

 could not be harmonized with what is known of the 

 mesoblast formation in the earthworm, the leeches, and 

 many other forms. I have recently been able, however, 

 to examine the development of an annelid {Nereis) 

 which I believe solves the puzzle and shows how Lopa- 

 dorhyncJms is connected with the other forms. The 

 early trochophore seems to be diploblastic, as in Lopa- 

 dorhyiicJins — i.e. to consist of ectoblast and entoblast 

 only, without any trace of mesoblast. In later stages 

 the mesoblast arises from the anterior ventral portion of 

 the outer layer (''ventral plate"). A study of the early 

 stages of development however — which Kleinenberg did 

 not succeed in following — shows that the cells of the 

 ventral plate are differentiated from the remaining outer 

 layer cells almost from the beginning of development, 

 and even without the use of reagents can be easily distin- 

 guished from the remaining outer layer cells in the fully 

 established ''diploblastic " trochophore. These cells have 



