Localization of the Nemertine Egg. 299 



4. Second polar body to beginning of the lateral elongation of 

 the egg. 



5. Elongated egg to the completion of the-first cleavage. 



6. Two-cell stage. 



7. Four-cell stage. 



8. Eight-cell stage. 



9. Sixteen-cell stage. 

 10. Blastula. 



The limits of these periods are fairly well given in their titles 

 and further definition is added later under each head. It may 

 be stated that in every case where the operation was performed 

 after fertilization the sperm had been added to the eggs approxi- 

 mately half an hour after removal of the latter from the animal 

 so that the first polar spindle was already in the metaphase in all 

 cases before the entrance of the spermatozoon. This treatment 

 gave a greater uniformity to the relations of maturation and fer- 

 tilization than would otherwise have been possible and the exter- 

 nal evidences of internal change as given by the polar bodies serve 

 as good landmarks. The various groups of experiments include 

 both the cases observed for the cleavage factors and those for the 

 morphogenic factors. Those on the unsegmented egg were de- 

 signed mainly to bring out the cleavage factors; those on the two 

 and four-cell stages were intended both for cleav^age and morpho- 

 genic factors; while those on the eight and sixteen-cell stages and 

 blastulae were designed wholly for the morphogenic problems. 



I. Fragments of Unfertilized Eggs. The cuts in this case 

 were made at periods ranging from a half hour to one and a half 

 hours after removal from the mother animal, the eggs being al- 

 lowed to lie in a dish of sea-water in the interval. Twenty-one 

 specimens were operated on, the cuts being made in the three 

 planes shown in Figure 3. Six of the cuts were horizontal, four- 

 teen vertical and one oblique. 



The localization of cleavage factors (Figures 4A, B, C). In 

 neither of the three groups was there an indication of a localiza- 

 tion of the cleavage factors. The fragments segmented in the 

 regular manner described for normal whole eggs. There is no 

 cross furrow, or only a very short one, and the normal rhythm, 



