Localization of the Nemertine Egg. 313 



In one of the two remaining cases the blastomeres were not 

 completely separated. The result was two connected partial em- 

 bryos in the early stages, which evidently later fused to form a 

 single individual. The resulting larva (age 33 hours) shows a 

 large blastocoele, two apical organs and a solid enteric mass grow- 

 ing in at the base. The blastocoele contains free rounded cells, 

 and there are no lappets (Fig. 13G). 



Finally there is the one case which was allowed to develop for 

 a sufficient length of time (47 hours) to give the organs a chance 

 to differentiate. The resulting larva (Fig. 13H) rotated rapidly 

 in life. It has a large blastocoele, a small enteron, an apical plate 

 and a thickening in the wall at the side of the mouth opening, 

 probably representing the basis of the ectodermal invagination at 

 this point. There are no lappets. With the exception of the small 

 size of the enteron and the absence of the lappets, the larva does 

 not differ widely from a normal larva. 



Summary of the results on the localization of morphogenic fac- 

 tors. The larvae developed from isolated blastomeres of the two- 

 cell stage do not show any constant defects except possibly as re- 

 gards the lappets, organs which in C. marginatus are developed 

 at a comparatively late period. Of the instances here cited only 

 two can be considered as old enough to have formed the lappets. 

 At any rate we must consider the larva developed from an iso- 

 lated blastomere of the two-cell stage to be retarded in develop- 

 ment as compared with a normal one of the same age, though this 

 view does not serve to explain completely the characteristics of 

 several of the larvae. 



VII. Four-cell stage. The experiments at this period come 

 under two heads. In one series the segmenting egg was divided 

 into two groups of two cells each, and in the other the four blasto- 

 meres were isolated. 



The isolated blastomeres segment in every respect as quadrants 

 of the whole egg. It will be remembered that the whole egg of 

 Cerehratulus goes through a definite twenty-eight-cell period be- 

 cause one of the cells of each quadrant lags behind the others in 

 its division as the egg passes from sixteen to thirty-two cells (see 

 Fig. iG). Correspondingly the isolated blastomere of the four- 



