8 . MARGARET MORRIS 



divide simultaneously, as in figure 22, so that the egg passes at 

 once from a 2-cell to a 4-cell stage. 



The normal cleavage is not given here in detail beyond the 

 4-cell stage. A 5-cell stage (not illustrated) results from the 

 division already begun in the egg illustrated in figure 25. The 

 cell di which is cut off from D is about the size of ^, jB or C, 

 so that there are four equal cells forming a cap on the cell D 

 which is still larger than any of them. Four hours after fer- 

 tilization the egg is in the condition shown in figure 26. Th^ 

 large cell is a descendant of D, the smaller ones of A, B, C, and 

 di. Figure 27 shows a late cleavage stage in which the lineage 

 of the cells cannot be traced. 



The 24-hour larva is sketched in surface view in text-figure 2, 

 1, The larvae are lively swimmers at this time, and it is diffi- 

 cult to get a drawing of a living specimen which shows more 

 than the general size and shape. The section (Fig.. 28) shows, 

 however, that the mid-gut, stomodeum, and shell-gland are 

 already formed, and that scattered mesenchyme cells are pres- 

 ent in the segmentation cavity. 



IV. DETAILED STUDY OF EXPERIMENTS 



a. Best method of inducing artificial parthenogenesis. As has 

 been said, many variations were made in the treatment by which 

 artificial parthenogenesis was induced. The most significant 

 variations are those in the exposure to heat, and the experi- 

 ments may therefore be grouped according to the temperature to 

 which the eggs were exposed. Variations in the hypertonicity 

 of the sea-water used as a second treatment and in the length 

 of the exposure to this agent seem to be of comparatively 

 slight importance, and it makes little difference in the result 

 whether the eggs are used as soon as they are shed or allowed 

 to stand for some time, first. The length of the interval be- 

 tween the heating and the treatment with hypertonic sea-water 

 is also unimportant. 



For counting the percentage of eggs that develop to larvae the 

 following method was used : Five hundred eggs were picked out 

 at random from the culture and set aside, and the number of 



