ORGANIZATION AND CELL-LINEAGE OF ASC1DIAN EGG. 99 



D. Genesis of the Ohganization of the Egg. 



It is probable that the differentiations of egg cells, of blastomeres, and possibly 

 t)f 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 ; in 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. In 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 Organisation and the 



Nuclear Inheritance Theory. 



The 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 

 1) 'tore 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 

 aria, 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 



