328 



INVERTEBRATA 



CHAP. 



being slightly larger. The third quartette is formed as usual at the 

 next cleavage ; 3d is larger than its sisters, and entirely composed of 

 white material. 



After this cleavage gastrulation begins by the macromeres passing 

 bodily into the blastocoele, just as in Patella. Of the fourth quartette 

 4d alone was clearly observed ; it is smaller than 3d and very much 

 smaller than 2d, and is pure wliite. 



As in Patella, cilia are developed about ten hours after fertilization, 



FIG. 257. Further stages in the cleavage of the egg of DentuUunt. (After Wilson.) 



A, beginning of third cleavage (8-blastomere stage), seen from the lower pole. B, the formation of 

 the second quartette of mieromeres, seen from the lower pole. The greater part of the substance which 

 formed the polar lobes passes into 2d. C, the formation of the third quartette of mieromeres, seen 

 from the lower pole. D, the division of the macromere which gives rise to the mother cell of the meso- 

 derm. j>>, third polar lobe. 



and in twenty-four hours well-developed Trochophore larvae are set free. 

 These are remarkable for their very broad prototroch, which consists 

 of three complete circles of large cells with cilia. The pre-trochal 

 region is short and conical ; it is covered all over with short cilia, and 

 it bears at its apex an apical plate with a long tuft of motionless 

 but flexible cilia. The post-trochal region is also short and conical, 

 and at its posterior end there is a telotroch consisting of a tuft of 

 short rigid hairs. 



The stomodaeum has not yet opened into the gut. This latter 

 consists of a sac-like stomach and a short blind intestine ; the anus 



