268 



MEAD. 



[Vol. XIII. 



that the primary prototroch is the same as in Amphitrite and 

 Clynienella. Figs. lOO, lOi show that the further divisions of 

 the cross cells correspond in direction exactly to those in Amphi- 

 trite, while other ectoderm cells conform to the law of alter- 

 nating oblique cleavage. 



I have been unable to discover the origin of the mesoderm 

 in Lepidonotus ; it may come from d^ as in other forms, but d*^ 



cannot be distinguished from 



a*, b*, and t", and all of these 



cells invaginate together with 



aj. \^ A„ B„ C„ D,. In the typical 



Fig. XIX, — Lefidonotus, very young trocho- 

 phore; ap.tft., apical tuft of cilia; prot., 

 prototroch ; the arrow indicates the direc- 

 tion of rotation. 



Fio. XX. — Lepidonotta, left side, second 

 day; ap.ift., apical tuft; frot., proto- 

 troch; mth., mouth. 



gastrula (Fig. 104), between the entoderm and ectoderm, cells 

 are sometimes found which are apparently mesoblastic in origin. 

 During the gastrula stage the trochophore assumes a remark- 

 able form, seen in text Fig. XIX. The membrane stands out 

 from the body except at the regions of the prototroch and 

 apical tuft. A much later larva is shown in text Fig. XX — 

 a form maintained for several days. 



Influence of temperature, direct heat, and light. — The in- 

 fluence of the temperature of the water upon the rapidity of 

 the cleavage processes is very pronounced. The difference of 



