330 



INVEETEBEATA 



CHAP. 



EXPEEIMENTAL EMBRYOLOGY OF DENTALIUM 



This peculiar development offers abundant opportunity for 

 experiment, as Wilson was not slow to perceive. Some of these 

 experiments were quite similar to those which he performed on 

 Patella, and led to similar results ; but the most interesting results 

 were those obtained by removing the polar lobe, which can be 

 readily done by means of a fine scalpel. When this is done at the 

 time of the first cleavage the embryo continues to develop, but all 

 the cells at the second cleavage are equal in size and possess no 

 lower white area, and no polar lobe is subsequently formed. 



At the subsequent cleavages the micromeres given off in the 

 D quadrant are precisely similar in size to their sisters, and the 



Fig. 2t)0. — Veliger larvae of ZlewfeiZMm. (After Wilson.) 



A, Veliger larva, thirty-two hours old. B, Veliger larva, three days old. Letters as in previous figure. 



In addition, /, foot ; m/, mantle fold ; sli, shell ; V, velum. 



embryo becomes a larva with a normal prototroch and a conical 

 pre-trochal region ; but there is no projecting post-trochal region, the 

 posterior surface of the larva being almost flat. The pre-trochal 

 region is covered, as normally, with fine cilia, but the apical tuft is 

 absent, and so is the thickened apical plate which is present in 

 normal larvae. On the other hand, the lateral ingrowths of ectoderm, 

 which we suppose to represent the cerebral ganglia, are present. 



Such larvae live four days and then disintegrate. Occasionally 

 a post-trochal protuberance appears to be formed, but when this is 

 examined by sections it is seen to be a plug of solid endoderm pro- 

 jecting through the open blastopore. No mesodermal bands are ever 



