144 Organisms from Eggs 



a nearly typical metameric seta-bearing trunk region, the 

 active movements of which show that the muscles are 

 normally developed. The pre-trochal or head region bears 

 an apical organ, but is more or less asymmetrical, and, in 

 every case observed, but a single eye was present, whereas 

 the normal larva has two symmetrically placed eyes. The 

 development of the anterior cell contrasts sharply with that 

 of the posterior. This embryo likewise produces a proto- 

 troch and a pre-trochal region, with an apical organ, but 

 produces no post-trochal region, develops no trunk or seta?, 

 and does not become metameric. Except for the presence 

 of an apical organ, these anterior embryos are similar in 

 their general features to the corresponding ones obtained 

 in Dentalium. None of the individuals observed developed 

 a definite eye, though one of them bore a somewhat vague 

 pigment spot. 



This result shows that from the beginning of develop- 

 ment the material for the trunk region is mainly localized 

 in the posterior cell; and, furthermore, that this material 

 is essential for the development of the metameric struc- 

 ture. The development of this animal is, therefore, to 

 this extent, at least, a mosaic work from the first cleavage 

 onward — a result that is exactly parallel to that which I 

 earlier reached in Dentalium, where I was able to show that 

 the posterior cell contains the material for the mesoblast, 

 the foot, and the shell; while the anterior cell lacks this 

 material. I did not succeed in determining whether, as 

 in Dentalium, this early localization in Lanice pre-exists in 

 the unsegmented egg. The fact that the larva from the 

 posterior cell develops but a single eye, suggests the possi- 

 bility that each of the first two cells may be already specified 

 for the formation of one eye; but this interpretation remains 

 doubtful from the fact that the larva from the anterior 

 cell did not, in the five or six cases observed, produce any 

 eye. 



