Localization of the Nemertine Egg. 325 



think the apparent contradictions may be explained in the fol- 

 lowing way, though the purely speculative character of all the 

 discussions is always to be kept in mind. 



Numerous recent observations, especially those of Lillie and 

 Conklin, indicate that cleavage is an accurate means of separating 

 materials already localized. My experiments on the eight-celled 

 stage of Cerebratiihis show that the first localization of materials 

 is in an apical-basal (polar) direction. It is probable, therefore, 

 that at the four-cell and even at the two-cell stage this same ten- 

 dency is the predominant one, so that at the four-cell stage there 

 are four equivalent parts and at the two-cell stage two equivalent 

 parts. However, in each of these two parts (taking the two-cell 

 stage as an example) there is an apical-basal differentiation. A 

 separation of a blastomere at this stage causes at first a half cleav- 

 age, but the materials retain a relation to each other very similar 

 to the normal whole relation as far as the apical-basal axis is con- 

 cerned along which differentiation is assumed; the embryo, there- 

 fore, can readily adjust itself to form a whole larva, having all 

 the necessary materials present in the proper relations. 



In the fragment of the unsegmented egg this is not true. Here, 

 according to all indications, there is a great activity in the mate- 

 rials of the egg. If the egg is cut at an early stage (as in the un- 

 fertilized egg) there is yet a considerable period of activity and 

 movement of materials through which the egg must pass before 

 the first cleavage takes place ; and, therefore, on the one hand a 

 whole cleavage results, and on the other a normal whole larva 

 is formed. Such is not the case, however, if we take the egg for 

 instance after maturation not long before the cleavage. The egg 

 is nearly ready for the first cleavage, the materials are arranging 

 themselves for an equal distribution and the proper physical ten- 

 sions for such a division are present. The egg is now cut and a 

 portion of it removed. The cleavage ensues very quickly, for the 

 physical machinery was already starting to act when the cut was 

 made. The different materials are not separated in a precise way, 

 even if the cut is vertical, for the cytoplasm is semi-liquid, and 

 the materials, especially along the injured side, come into abnor- 

 mal relations with each other, which cannot be regulated as in 



