256 GLA VPOLE. [VoL. XIV. 
taining the numerically established ratio of cells (Fig. 35). The 
next change is that in a certain place the blastoderm cells cease 
to divide, and assume certain definite characters. This is the 
beginning of the “‘ precephalic organ,”’ or, as it has been called, 
the “dorsal organ.’ Figs. 35-39 illustrate the stages by 
which the organ attains its full size. After the nuclei cease to 
divide, the cytoplasm begins to increase in amount and becomes 
highly vesicular in structure, forming a thick layer (Fig. 35). 
The nuclei increase in size, but do not divide either kinetically 
or akinetically. These changes -apply only to the ectodermic 
cells. The mesoderm cells lying below these disappear gradu- 
ally, partly by migration and partly by disintegration. At first 
the protoplasm accumulates more rapidly below the ectodermic 
nuclei, placing these on the periphery, but soon a process of 
infolding and insinking begins. The nuclei gradually sink lower 
as the amount of protoplasm increases, until a condition shown 
in Fig. 37 is reached; this is quickly followed by the stage of 
sreatest development shown in Fig. 38. Here the precephalic 
organ (#c.o.) resembles a large gland; on a whole egg it appears 
as a large, circular, lighter mass that is clearly of some depth. 
By dissecting it out, the organ is found to have the form of an 
oblate spheroid, as would be inferred from its form in sections. 
During the development of the organ by a process of invagina- 
tion, the surface over which it reaches is much reduced (Figs. 
36-38), but the number of cells remains the same. The neces- 
sary crowding down of the nuclei in this process causes them 
to remain at different levels, suggesting that the organ is com- 
posed of several layers. This is, however, simply an appearance, 
there being only one layer of cells and these ectodermic. The 
cells always remain distinctly separated from each other, and 
the protoplasm is very vesicular. 
The next change is a striking one; the vesicular character of 
the protoplasm is supplanted by a strongly marked striation that 
appears first at the outer edge in vertical planes. The final 
result of this process is seen in Fig. 39, where the nuclei are 
crowded to the bottom of the organ, which is irregular in shape 
and apparently beginning to degenerate. The outer ends of 
the cells have been elongated and drawn out into fine threads, 
