Decembee 2, 1904.] 



SCIENCE. 



749 



that of the isolated anterior cell, especially 

 in the genus Lanice, which I have found 

 the most favorable egg for operation of all 

 the many forms tested. While the number 

 of successful operations has been rather 

 small, they have given a very definite re- 

 sult, and one that is in its main features in 

 agreement with the one earlier obtained 

 in the eggs of the mollusk Dentalium. 

 When either cell of the two-cell stage is 

 destroyed, the remaining cell segments as 

 if it still formed a part of an entire em- 

 bryo. The later development of the two 

 cells differs in an essential respect, and in 

 accordance with what we should expect 

 from a study of the normal development. 

 The posterior cell develops into a seg- 

 mented larva with a prototroch, an asym- 

 metrical pre-trochal or head region, and a 

 nearly typical metameric seta-bearing 

 trunk region, the active movements of 

 v/hich 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 pos- 

 terior. This embryo likewise produces a 

 prototroch and a pre-trochal region, with 

 an apical organ, but produces no post- 

 trochal region, develops no trunk or setse, 

 and does not become metameric. Except 

 for the presence of an apical organ, these 

 anterior embryos are similar in their gen- 

 eral features to the corresponding ones ob- 

 tained in Dentalium. None of the indi- 

 viduals observed developed a definite eye, 

 though one of them bore a somewhat vague 

 pigment spot. 



This result shows that from the beginning 

 of development 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 structure. 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 preexists 

 in the unsegmented egg. The fact that the 

 larva from the posterior cell develops but 

 a single eye suggests the possibility that 

 each of the first two ceUs 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 ob- 

 served, produce any eye. This subject 

 remains for further examination, which I 

 hope to be able to carry out hereafter. 



These observations, in connection with 

 my preceding ones on the molluscan egg, 

 contribute to the growing body of evidence 

 that the development both of annelids and 

 of mollusks is to be regarded as a mosaic- 

 work of self-ditferentiating cells ; and, fur- 

 ther, that the specific morphogenic factors 

 are connected in some way with specific 

 forms of protoplasm, which I think may 

 conveniently be designated as formative 

 (perhaps better, 'morphoplasmic') stuffs. 

 I have endeavored in two recently pub- 

 lished papers to show that the same prin- 

 ciple applies to the eggs of other animals, 

 even in such cases as the sea-urchins or 

 Amphioxics, where either of the first two 

 cells may produce a complete embryo. The 

 explanation of the difference between the 

 two cases appears to be that in the mollusk 

 or annelid the cytoplasmic stuffs undergo 

 an asymmetrical distribution during the 

 first division, while in the sea-urchin or 

 Amphioxus the distribution is symmetrical. 

 The division is, therefore, in the first case 



