ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 99 
D. GENESIS OF THE ORGANIZATION OF THE EGG. 
It is probable that the differentiations of egg cells, of blastomeres, and possibly 
of all types of cells, are reducible to two fundamental processes :—(1) the genesis of 
unlike substances, and (2) the localization of these substances in definite parts. 
Few observations or experiments have been made on the former of these processes 
and probably no other problem of development would better repay a thorough 
investigation ; the localization problem has been approached from many sides and 
has yielded results of great interest and importance. 
It is a significant fact that localization in the unsegmented egg takes place in 
so many cases at the time of maturation and fertilization. This is the case in cer- 
tain ascidians, fresh-water snails, nemerteans and echinoderms; іп Myzostoma and 
Dentalium the two poles of the egg are dissimilar while the egg is still in the ovary, 
but here also active localization goes on during maturation. In ascidians and 
fresh-water snails it is not possible to determine whether the movements which 
lead to localization are dependent upon the maturation or upon the fertilization of 
the egg, since as yet it has not been possible to separate experimentally these 
processes; they certainly seem to be associated with the entrance of the sperm- 
atozoon, but since the maturation does not here occur until after the fertilization, 
it is not possible to determine with certainty the relative importance of these two 
processes in causing localization, Іп Strongylocentrotus the movements which 
lead to the formation of the red pigment zone occur after the extrusion of both 
polar bodies and before fertilization; in this case therefore the localization is 
associated with the maturation. 
1. Role of the Nucleus in Differentiation; Cytoplasmic Organization and the 
Nuclear Inheritance Theory. 
Тһе localization which is effected in the ascidian egg upon the entrance of the 
spermatozoon is by no means the initial localization in this egg. In the ovocyte 
before maturation and fertilization the mesoplasm, which later give rise to the meso- 
derm, exists as a peripheral layer of protoplasm, the ectoplasm, which in later 
stages is chiefly distributed to the ectoderm, is in large part contained within the 
germinal vesicle, while the yolk-laden portion of the egg, the endoplasm, which 
later passes largely into the endoderm, is nearly central in position (figs. 61, 76). 
At an earlier stage neither the peripheral layer nor the yolk are recognizable as 
such ; the cell body is composed of granular deeply-staining protoplasm, and around 
the nucleus is a distinct granular mass, the “ yolk matrix” of Crampton (1899). 
In the very young ovocyte this granular mass is situated chiefly on one side of the 
nucleus, and frequently contains at its center a large granule, surrounded by a clear 
area, which I take to be the centrosome; the granular mass surrounding this is 
accordingly sphere material or archoplasm. 
In the growth of the ovocyte the sphere material enlarges and spreads around 
the nucleus, forming the yolk nucleus or matrix; it then begins to disintegrate into 
