ix MOLLUSCA 307 



aiding the effect of the water either by shaking or cutting the larva 

 with a scalpel. 



An isolated blastomere of the 2 and 4-cell stages was found to 

 be exceedingly difficult to rear ; it always segmented as if it formed 

 part of a complete embryo. In most cases the resulting mass of cells 

 flew into pieces without producing anything which could be called 

 a larva. In a few cases, however, this disruption did not happen, 

 and then there resulted a larva of diminished size and of the closed 

 or open type. Such larvae may be termed closed, or open dwarf 

 larvae respectively. By the latter term is meant a larva in which 

 the half or' quarter ectoderm does not cover the whole egg, by the 

 first term a larva in which, by secondary shiftings of the cells, the 

 macromeres are entirely covered by ectoderm. In the closed type of 

 larva, in spite of these secondary shiftings, each cell undergoes the 

 fate which would have befallen it had it formed part of a perfect 

 egg ; thus a larva developed from a single blastomere of the 2-cell stage 

 produces only two groups of primary trochoblasts, and a larva from a 

 blastomere of the 4-cell stage only one. It is obvious that in 

 these closed larvae, each cell forms a larger part of the periphery of 

 a sphere than it would normally do; and it must, therefore, be 

 subjected to a considerable strain in order to produce such an 

 abnormally sharp curvature, and this strain may account for the 

 explosive character of these dwarf larvae. These larvae, if they live, 

 always finally " close," and then gastrulation occurs : they always 

 produce an apical organ. 



Much greater success attended Wilson's efforts to separate the 

 ruicromeres of the 8-cell stage. These when isolated segmented 

 exactly as if they still formed part of the whole egg ; at the end of 

 twenty-four hours they were converted into little ectodermic vesicles, 

 with an apical organ at one end and a group of four primary trocho- 

 blasts with their long powerful cilia at the other (Fig. 243, A and B). 

 When the entire group of the first four micromeres were isolated 

 they also segmented as if they still formed part of the entire egg ; 

 but they proved to be a very unstable combination, some cells always 

 separated, and the largest dwarf larva that was obtained represented 

 the products of the division of only three micromeres. 



When the products of the division of the first quartette of 

 micromeres were isolated, similar results were obtained. Such cells 

 always segmented as if they formed part of the entire egg, and later 

 endeavoured to round themselves off and form ectodermic vesicles. 

 Thus, cells belonging to the group Iq 1 developed into ectodermic 

 vesicles with an apical organ at one end and two secondary trocho- 

 blasts at the other; these secondary trochoblasts being at once 

 distinguishable from the primary ones by the smaller size of their 

 cilia. Cells belonging to the group Iq 2 divided into a group of four 

 primary trochoblasts; cells belonging to the groups Iq 21 or Iq 22 

 divided once and produced a pair of trochoblasts ; cells of any 

 of the groups Iq 211 , Iq 212 , Iq 221 , or Iq 222 , when isolated, did 'not 



