No. I.] THE EMBRYOLOGY OF CREPIDULA. 63 



recently Erlanger ('92) has reached the same view concerning 

 Bythinia. Neither of these cases, however, is conclusive, and 

 I have little doubt that a careful reexamination would show 

 that here also three, and only three, quartettes of ectomeres are 

 formed. 



No phenomenon in the whole history of cleavage seems to 

 me more remarkable than this. As just said, it occurs almost 

 universally among mollusks and annelids, in equal or unequal 

 cleavage, and in eggs varying in size from a few microns to 

 more than a millimeter in diameter. Associated with it is the 

 formation of the mesoblast and entoblast in all these forms in 

 the fourth quartette. The cause of this remarkable phenomenon 

 is to be fotmd in the fact, as I believe, that each of these quar- 

 tettes of ectomeres is the protoblast of definite regions and 

 organs of the larva. In all cases in which three quartettes of 

 ectomeres are formed, the first quartette gives rise to all the 

 umbrella region and at least a portion of the prototroch ; the 

 second quartette gives rise to the median anterior, posterior, 

 right and left portions of the body ; while the third quartette 

 gives rise to the regions intermediate between those formed by 

 the second quartette. 



In Umbrella the different quartettes are successively larger, 

 the first being smallest and the fourth largest. In Crepidula 

 the difference in size between the first three quartettes is very 

 slight, though the second quartette is perhaps somewhat larger 

 than either the first or third ; the fourth quartette, owing 

 chiefly to the amount of yolk which it contains, is very much 

 larger than either of the preceding ones. In general the rela- 

 tive size of the different quartettes of ectomeres depends upon 

 the relative size of the regions and organs of the larva to which 

 they give rise, and also upon the relative time at which these 

 organs are formed. 



5. Division of the Second Quartette of Micromeres . Figs. 18, 

 ig, Diagram /f.. 



Although it is not my purpose to take up the history of the 

 micromeres until after I have described the complete segrega- 

 tion of the layers, it seems best in this section to trace the 



