Localization of the Nemertine Egg. 315 



zation. The larvae resemble very much the solid larvae of the 

 isolated blastomeres of the two-cell stage, but are of smaller size 

 (Fig. 14E). All the one-fourth larvs are like the one figured. 

 There is a solid archenteric growth, but no sign of an apical organ. 



The larvae from isolated blastomeres of the four-cell stage, 

 therefore, give no indication of a definite localization of the mor- 

 phogenic factors, though they do not develop in an entirely nor- 

 mal manner. The latter statement holds also for the larvae de- 

 veloped from the isolated blastomeres of the two-cell stage, as has 

 already been stated (p. 311). 



VIII. Eight-cell stage. The results yielded by the experiments 

 on this stage are perhaps the most important of all those given. 

 The group of experiments included sixty-eight four-cell groups. 

 These groups were separated by a careful cut with the fine scalpel 

 blade used in all the experiments. In most cases the knife blade 

 passed between the cells, and the latter were entirely uninjured by 

 the operation. In a few, however, the protoplasm was cut, and 

 these will be mentioned in the descriptions. The operations in- 

 clude a series of horizontal cuts separating the upper from the 

 lower quartet, and a series of vertical cuts separating the two 

 lateral four-cell groups, each of the latter containing two cells of 

 the upper and two of the lower quartet. There are thus three 

 kinds of four-cell groups, the larvae from which are to be com- 

 pared: (i) Upper quartets, (2) lower quartets, and (3) lateral 

 four-cell groups. The experiments yield a very definite and posi- 

 tive result. The larva developing from the upper quartet have 

 an apical organ, but no archenteron, those from the lower quar- 

 tet have an archenteron, but no apical organ, while those from 

 lateral four-cell groups have both apical organ and archenteron. 



The natural conclusion '4o be drawn from these results is that 

 certain organ-forming materials are definitely separated by the 

 third cleavage plane, and the larvae developing from the lower 

 or the upper quartet have not the power of making up the lack- 

 ing material. The lateral four-cell groups, howev^er, possess both 

 kinds of materials and are, therefore, able to develop both archen- 

 teron and apical organ, though the larvae are usually asymmet- 

 rical. , 



