No. 3.] THE CELL-LINEAGE OF NEREIS. 439 



3. Ge7ieral Comparisons with Other Animals. 



It is not my purpose to undertake an extended comparison of 

 the cleavage with that of other animals, but there are certain 

 resemblances so striking in themselves, and so interesting when 

 carefully analyzed, that it is impossible to pass them by. These 

 resemblances relate especially to the cleavage of the molluscan 

 and the polyclade ovum, and they are of such importance that 

 I give a number of diagrams to set them clearly forth. For the 

 sake of condensation I shall pass by most of the earlier litera- 

 ture and select as types the development of the Polyclade, 

 Discoccelis, as described by Lang (No. 15), of the gasteropod 

 Neritina (Blochmann, No. 2), and of the gasteropod Crepidnla 

 (Conklin, No. 4).! 



Up to a late stage in the spiral period {tzventy-eight cells) every 

 individual blastomere and evejy cell-division is represented by 

 a corresp07iding blastomere and a con^esponding cell-division in 

 the embryo of the polyclade, and in that of the gasteropod. In 

 all three the first two cleavages and the upper and lower cross- 

 furrows have the same relations. In all, three groups of four 

 micromeres each are successively separated from the macro- 

 meres, — the first group in a right-handed spiral, the second 

 in a left-handed spiral, and the third in a right-handed spiral, 

 like the first. The micromeres of the second and third groups 

 alternate with one another so as to form an outer belt of eight 

 cells that surrounds the four primary micromeres (Diagram 

 VII, A, B). 



In all, the primary or central micromeres likewise undergo 

 three spiral cleavages, the first right-handed, the second left- 



1 I much regret that I cannot make full use of Conklin's beautiful studies of the 

 gasteropod cleavage, since they have not yet been published in full. The work has 

 been carried on at Wood's Holl at the same time with my own studies on Nereis ; 

 and to Professor Conklin's courtesy I owe the opportunity to examine his entire 

 set of unpublished drawings, and to verify their accuracy in respect to some of the 

 most important stages by the study of the original preparations. This is by far the 

 most accurate and complete study of the molluscan cleavage that has thus far been 

 made. The resemblance to the Nereis cleavage is so close and extends over so long 

 a period as to be most extraordinary. Since, however, the figures may not be pub- 

 lished for some time, I am compelled for the present to take Blochmann's Neritina 

 as my principal basis of comparison. I may add that, in the course of the past 

 summer, Conklin has ascertained that the first cleavage-plane in Crepidnla is not 

 longitudinal, as he at first described it, but transverse, precisely as in N'ereis. The 

 preparations he has shown me seem to leave no room for doubt on this point. 



