34 LILLIE. [Vol. X. 



seren." The origin of the mesoblast, from two bilaterally 

 symmetrical cells, has often since been described. Professor 

 Whitman (No. 6i, 1878) was the first to show that there was a 

 perfectly definite cell history and origin from single cells for 

 other structures of the adult. He demonstrated that, in 

 Clepsine, not only the mesoblast, but the ventral nerve cord 

 and the trunk nephridia could be traced back to single cells ; 

 and further, that the cells representing these structures were 

 the product of the posterior macromere of the four-cell stage, 

 which was thus the representative in this stage of the whole 

 trunk. In 1879 Rabl's paper on Planorbis appeared. He 

 showed, for the first time, that the whole ectoderm was formed 

 in a series of three cleavages each, from four basal macromeres. 

 Blochmann (No. 35), 1882, described three generations of 

 ectomeres in Neritina, and derived the mesoblast from 

 the fourth cleavage of the posterior macromere. During 

 the last twelve years a great many papers dealing with the 

 cleavage have appeared. In this section I wish to point 

 out the most obvious results of the later work on the annelids 

 and molluscs, and to show the bearing of my observations 

 on Unio, 



As I have already said, Rabl was the first to show for any 

 form that the epiblast is formed in three generations of ecto- 

 meres from four basal macromeres. (Fol described these three 

 generations, but believed that the macromeres contributed to 

 the epiblast after that.) Since then the same thing has been 

 shown to be true for Neritina (Blochmann, No. 35), Umbrella, 

 (Heymons, No. 47), Crepidula (Conklin, Nos. 39 and 40), Limax 

 (Kofoid), and Unio among the mollusca ; and for Nereis limbata 

 (E. B. Wilson, No. 64), Nereis dumerilii (v. Wistinghausen, No. 

 66), Polymnia, Spio, and Aricia (E. B. Wilson, No. 64) and 

 Amphitrite (Mead) among the Annelids. ^ v. Wistinghausen 



1 McMurrich (No. 56) thinks that more than three generations of micromeres are 

 formed in Fulgur. He thinks that " probably the amount of yolk present influences 

 the number of spherules formed." It may, of course, be true that more than three 

 generations of micromeres are formed in Fulgur, just as in Polymnia and Aricia 

 (Wilson, No. 64, p. 458). The important point to determine is how many of these 

 generations are ectomeres. From the recent studies it seems to follow that while 

 the number of generations of micromeres is variable, the number of generations 



