Ds EDWIN G. CONKLIN 
DiS Generaliresultsiot thesevexpeninienits eee ricer 75 
i Nuclear'sizean centritugediegosew a err ree chee ie 
2. The sizes of spindles, centrosomes, spheres and asters........ 77 
3. The rhythm of division in centrifuged eggs.................... he 
4. Growth of cytoplasm at the expense of yolk................... an 
5. Unequal and differential cell divisions....................... _ the 
65 Regulationanathercleavacelprocessas.- nie 81 
General summary andindex? §266 See ee ee 83 
Literature cited. ¢).yei7 30 Ses. 36 tedster eee ee 88 
In the development of all organisms considerable differences 
of size appear, sooner or later, among constituent cells; sometimes 
the blastomeres of the cleaving egg differ in size, in other cases 
these differences appear only later during the blastula, gastrula, 
or larval stages. Endoderm cells are usually larger than those 
of the ectoderm, ciliated cells are generally larger than non-ciliated 
ones, muscle and nerve cells are usually larger than epithelial 
or mesenchyme cells. 
These differences in the size of cells may be due to unequal cell 
division, to unequal rate of division, or to unequal growth of 
cells after division, and in some cases all of these factors may be 
represented in the same egg or embryo. It is frequently assumed 
that unequal cell divisions are caused by the accumulation of 
metabolic substances, such as yolk at one side of a cell, and the 
crowding of the protoplasm and nucleus to the opposite side. 
Such unequal divisions are frequently found in yolk-laden eggs, 
and may be artificially produced at will by centrifuging the yolk 
to one pole or the other of a dividing cell. But in many cases this 
is not the cause of unequal cell division; the yolk may be uniformly 
distributed with regard to the poles of the spindle and yet the 
cleavage may be unequal, or unequal division may take place in 
purely protoplasmic cells, in which the eccentric position of the 
spindle is not due to pressure. Innumerable cases of this sort 
have been found both in normal and in experimentally altered 
conditions. Often such unequal divisions are associated with 
visible histological differences in the resulting cells. The study 
of cell-lineage has shown that in some cases a particular cell is 
distinguished from the time of its formation by its size, proto- 
plasmic structure, rate of division, prospective significance and 
