254 



BIOLOGICAL LECTURES. 



in the case of Crepidida and many other gasteropods, since here 

 the " left posterior macromere is not appreciably larger than the 

 right, and in some {e.g., Umbrella, Heymons, '93) it is smaller, 

 and . . . the mesoblast {f\d) is only one member of a quartette 

 which is separated in a left spiral from the macromeres, each 

 of the other members being quite as large, or even larger than 

 the cell 4<^." And finally, after consideration of the manner in 

 which bilateral cleavages arise, he says : " The conclusion, there- 

 fore, is unmistakable that bilaterality first appears in processes 

 which lead to the formation of the trunk and the elongation of 

 the future animal, while the primitive radial symmetry of the 

 anterior quadrants is correlated with the fact that these quad- 

 rants give rise largely to larval organs, most of which bear 

 traces of radial symmetry." And again it must be said of this 

 conclusion, as Conklin said of Wilson's, that it is not applicable 

 here, or applicable only in modified form, for the anterior quad- 

 rants show a high degree of bilaterality in many forms. 



I believe the facts as known thus far indicate that the func- 

 tion of bilateral cleavage is tJie symmetrical distribiition of the 

 material for tJie formation of the adult or larva. It is evident 

 that in the adaptation of means to end symmetrical divisions 

 would be useless at a stage when the material was not in a posi- 

 tion to be distributed. Suppose the radial divisions forming the 

 cross (Fig. 7) appeared before the cross cells were surrounded 

 by a complete ring of cells. The formation of the cross would 

 not result in forcing the ectodermal cap down over the &gg, 

 but would simply produce four lines of cells extending outward 

 from the anterior pole as far as or beyond the other ectoderm 

 cells. Again, suppose the first somatoblast 2d were to divide 

 bilaterally at its first division or its second : the growth of the 

 somatic plate in the direction which is to result in its concres- 

 cence would not be begun ; but, as it is, two cells are formed, 

 lying on either side of, and posterior to, the stem cell {Fig. 9), 

 and then a cell is given off anteriorly in the median line (Fig. 

 10), and thus the whole mass is forced somewhat posteriorly 

 and the symmetrical distribution of the material is begun. 

 Then in succeeding divisions of the stem cells (Fig. 11-17) 

 how exactly the relation between the parts is preserved, every 



