Localization of the Nemertine Egg. 3 1 1 



flat plate-like forms, there being all gradations between the em- 

 bryos curved into a deep cup through those showing only a slight 

 curvature up to perfectly flat plates (Figs. 12D to G and 13 A). 

 On the other hand there is a similar graded series from the cup 

 forms up to perfectly closed spherical half-embryos usually con- 

 taining a very small cavity or none at all (Fig. 12C). It seems 

 probable that these differences of form are the result of slight 

 changes in the surface tension relations between the cells, as Pro- 

 fessor Wilson has suggested, and this view is strengthened by 

 my observation of the development of a plate form and a cup 

 form from the two blastomeres of a single egg. 



2. Experiments on the localization of the morphogenic fac- 

 tors. The localization of the morphogenic factors in the two- 

 cell stage was not made out as fully as could have been wished. 

 The larvae were killed in most cases at too early a stage to deter- 

 mine the necessary difi^erentiation of organs. The two blasto- 

 meres were separated in each of sixteen eggs and in thirteen larvae 

 were obtained. Most of these were about 33 hours old when 

 killed, only three being older than this. The thirteen individuals 

 are divided into groups of similar cases in the following descrip- 

 tion. 



In three cases observations were made on the activity of the 

 embryos, but the embryos themselves were lost during transfer- 

 ence to the preserving liquid. An interesting fact in connection 

 with these, and this holds also for other one-half as well as one- 

 fourth embryos, is the abnormally great rapidity of rotation in 

 most of the cases. 



Another group is formed by isolated blastomeres from five eggs. 

 The larvae were distinguished by rapid rotation in life and by a 

 dense ingrowth of cells from one pole, which entirely filled the 

 blastocoele and came close up against the ectoblastic wall around 

 the whole surface of the egg (Fig. 13B, C, D). No apical plate 

 was made out in any of them, but in one case there was a single 

 lappet (Fig. 13B). 



