Experimental Studies on Germinal Localization. 239 



(from acetic-glycerine preparations). Of these twins one (Fig. 

 113) is closed and shows the gastrulation well advanced (the su- 

 perficial ectoblast-cells of the lower hemisphere are only in part 

 shown). This larva shows very clearly the two groups of pri- 

 mary trochoblasts on opposite sides of the egg, at t and t, with 

 at least two secondary trochoblasts lying between them on each 

 side, the general arrangement being similar to that shown in Fig. 

 116.^ The twin larva (Fig. 114) is still widely open on one 

 side; and while the small ectoblast cells have closed in to fill the 

 gap above, the two primary trochoblast groups lie at one side, 

 leaving a wide gap occupied by smaller cells. Fig. 115 is a J-^- 

 larva, which, though somewhat asymmetrical, is clearly of the 

 closed type (the superficial post-trochal ectoblast-cells are shown 

 only in optical sections at the sides) ; and here, too, the primary 

 trochoblast groups lie on opposite sides of the larva. 



It is hardly possible to doubt that these two types of larvae 

 arise from the open and closed types of cleavage, the trochoblasts 

 having undergone their normal differentiation whether displaced 

 or not. This has not been strictly proved by isolation experi- 

 ments ; but in view of the demonstrated fact that the trochoblasts 

 differentiate typically If wholly separated from their fellows, 

 there can be no doubt, I think, of the interpretation offered. It 

 is quite clear that in this case the prospective value of the cell is 

 not a function of its position, but is dependent on its internal or- 

 ganization irrespective of its position. This result is exactly anal- 

 ogous to those obtained by Fischel ('98) by displacing the micro- 

 meres of the ctenophore egg — an operation that, as he shows in 

 the most convincing manner, leads to a correspoiiding displace- 

 ment of the rows of swimming plates in the larva. 



With the M-larvas in Patella I had little success, since they al- 

 most invariably broke apart into smaller fragments. A very few 

 nearly complete larvae of 24 hours were, however, obtained, one 

 of which is shown in Fig. 112. This larva shows a central mass 

 of rather large rounded cells completely surrounded by ectoblast, 

 and has evidently gastrulated. At one side is a very distinct group 



iThe latter larva apparently shows five primary trochoblasts on one side 

 — a fact for which I cannot account. 



