Localization of the Nemertine Egg. 295 



sumed. This method of cutting was found to be very successful 

 for segmented eggs as well. Even at the eight-cell stage the upper 

 and lower or the lateral groups of fours may be separated in a 

 considerable percentage of cases without injury to the individual 

 blastomeres notwithstanding the interlocking of the cells. 



3. Normal Development. 



The orientation of the egg in Cerehratiihis marginatus, as in 

 C. lacteits, is made easy by the presence of a basal protuberance 

 before and for some time after fertilization and later by the pres- 

 ence of the polar bodies at the opposite pole. The protuberance 

 is still evident when the first polar body is formed, but usually dis- 

 appears at about the time of the formation of the second polar 

 body. A considerable difference was noted in the ability to with- 

 stand cutting at different periods. Before fertilization the egg 

 could be cut very readily, an extra-ovate being formed in relatively 

 few cases. After the first polar body had been formed, however, 

 the texture of the protoplasm seemed entirely different, the eggs 

 going to pieces in the great majority of cases immediately after 

 the cut was made. Again after the second polar body had been 

 formed the cutting property seemed much better, the quality of 

 the cut resembling that of the unfertilized egg. However, it is 

 very hard to draw any definite conclusion as to the comparative 

 texture of the eggs at these different stages because the amount of 

 flattening of the eggs on the slide, the sharpness of the scalpel 

 and practice in handling the latter may have had a great deal to 

 do with the cleanness of the cut. The general impression, leav- 

 ing out as far as possible these disturbing factors, is that the 

 protoplasm is much more liquid at the stage with one polar body 

 than that at either the unfertilized stage or the two polar body 

 stage. The maturation divisions seem therefore to be accom- 

 panied by a profound change in the nature of the cytoplasm. 



When the egg is first removed from the body of the animal 

 there is a large germinal vesicle. This is usually situated on the 

 polar axis of the egg near the side farthest from the basal pro- 

 tuberance (Fig. 3, p. 301). The outline of the germinal vesicle 



