124 



INVEETEBEATA 



CHAP. 



discovered, so that these larvae could be reared in large numbers 

 through their metamorphosis, under experimental conditions, and if 

 each stage in this change were thoroughly examined by sections, then 

 a flood of much-needed light would be thrown on this period of 

 Nemertine development. 



If the reader has followed the description so far given it will be 

 evident that when all four amniotic imaginations completely coalesce 

 they must cut the larva into an upper and a lower half. This is just 

 what happens ; and the lower and inner half, invested by the coalesced 

 floors of the amniotic invaginations, and containing the alimentary 

 canal, drops to the bottom of the sea and commences life as a young 



.pr 



aim 



-p. cm 



FIG. 99. Tsvo stages in the development of the Nemertiue rudiment within the Pilidium, 

 viewed from above. (After Salensky. ) 



c.s, cephalic slits ; o, moutli ; oes.p, oesophageal pockets. 



Nemertine worm. The upper half consisting of the larval ectoderm, 

 including the prototroch, lappets, and apical sense organ, and bounded 

 inside by the coalesced roofs of the invaginations or amniotic invest- 

 ment, continues to swim about for a little time before its energies are 

 exhausted, and then it dies. 



EXPERIMENTAL WORK. 



E. B. Wilson and his pupils Yatsu and Zeleny have performed a 

 most interesting series of experiments on the eggs and embryos of 

 Cerebratulus, the general results of which may be shortly recounted 

 here. The unfertilized egg was cut or shaken into fragments. If 

 this be done before the membrane of the nucleus has disappeared, and 

 if sperm be added to the fragments, only the fragment in which the 



