No. 3-] THE CELL-LINEAGE OF NEREIS. ^^g 



mass is caused by its growth as a whole, and is not a resultant 

 of the growth of the cells individually considered. This is 

 proved in plants by many facts, of which, perhaps, the most 

 obvious is the existence of multicellular forms, in which the 

 characteristic form of growth is fully determined before the mass 

 divides into cells. Sachs concludes that the fundamental law 

 of cell-division is such that the cleavage-planes primarily are 

 vertical to one another and either vertical or parallel to the 

 surface of the dividing mass ; and he shows by a masterly 

 analysis that the cleavage-planes in growing structures of 

 various forms conform very nearly to the theoretical mathe- 

 matical demands of this law. 



From an a priori point of view there is every reason to 

 believe that a like relation between growth and cell-division 

 exists in animals, although its demonstration is difficult, espe- 

 cially for the reason that the two processes so generally go hand 

 in hand. As Rauber points out, however, the segmentation 

 of the ovum is a case in which, very often at least, there is no 

 appreciable growth or change of form throughout a long series 

 of cell-divisions, the ovum having assumed its form and size 

 before the beginning of cleavage. 



I am but expressing a commonplace of embryology in stating 

 that the more nearly the ovum approaches its ideal form, the 

 sphere of uniform protoplasm, the more nearly do the first three 

 cleavages divide it into equal octants by division-planes cutting 

 each other at right angles ; in other words, the more nearly 

 do they conform to the theoretical requirements of Sachs's law. 

 It is interesting to find that the first three cleavages of such an 

 ovum as that of Amphioxus or Echinus are almost exactly dupli- 

 cated by the first three divisions of the spherical embryos of 

 ferns and phanerogams. These facts in themselves create a 

 strong presumption that plants and animals agree in the funda- 

 mental relations of growth and cell-division. If we examine 

 the first three cleavages more narrowly we find that two types 

 of division (apart from the bilateral) may be distinguished. The 

 first is what may be called the true radial type (represented 

 typically by AmpJiioxiLS and Echinus), in which the first two 

 cleavages are perfectly vertical and the third perfectly horizontal, 

 so that the four upper cells lie exactly above the four lower 

 cells respectively. The second is the spiral type, represented 



