102 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
2. Factors of Localization. a. Cytoplasmic Movements. 
Undoubtedly the most important of all the localizing factors so far recognized 
are cytoplasmic movements. Such movements have been observed in unsegmented 
eggs as well as in the cleavage cells, and they are generally associated with local- 
ization of unlike substances and frequently with cell division. The importance of 
such movements in the differentiations of the egg I first recognized in Crepidula 
(1899), where the movements of the cytoplasm during cleavage are very exten- 
sive. In the ascidian egg, on the other hand, these movements are most pronounced 
in the period between the fertilization and the close of the first cleavage. Іп both 
the aseidian and gasteropod these movements are definitely directed and bring 
about a constant and typical form of localization of the materials of the egg. 
The fact that these movements are definitely directed shows that they are de- 
pendent upon a constant organization of the cell; their immediate cause is unknown. 
So far as I have observed, these movements always begin soon after the disappear- 
ance of the nuclear membrane and the consequent escape of nuclear material into the 
cell body. Іп the case of the gasteropods, I have suggested (1902) that one of the 
characteristic movements of the telophase of division is due to the affinity of the 
sphere material for oxygen. After the formation of this sphere material, during 
each cell division, it moves to the surface of the cell and as nearly as possible to the 
animal pole. If, however, the eggs be placed in water, from which the oxygen has 
been removed by boiling, this movement to the surface does not take place. In the 
ascidian the entrance of the spermatozoon seems to be the inciting cause of the 
movement. Тһе peripheral protoplasm (mesoplasm) rushes down to the point of 
entrance and masses around the spermatozoon; then when the latter moves toward 
the posterior pole this protoplasm goes with it and is thus gathered into the cres- 
cent; finally, when the sperm nucleus moves in toward the centre of the есе the 
larger part of this protoplasm remains at the surface, while a small portion of it is 
drawn in with the sperm toward the center of the egg. Іп these movements, as well 
as in the subsequent ones during cleavage, the mesoplasm remains near the surface of 
the cell and in this respect resembles the sphere substance of the gasteropod egg. 
The flowing of the protoplasm to meet the entering spermatozoon is a phenome- 
non of rather general occurrence. Іп most cases this leads only to the formation 
of a small protoplasmic field around the sperm and sometimes to the formation of 
an entrance cone; in the ascidian practically all the protoplasm of the egg takes 
part in this movement leaving the maturation spindles with only a trace of pro- 
toplasm around them. This withdrawal of the protoplasm from the animal pole 
may be associated with the fact that there are no centrosome or asters in the 
maturation spindles, whereas there is a large centrosome and aster in connection 
with the sperm nucleus. Certainly the clear protoplasm is usually found in the 
region of the asters. What the exact nature of this attraction between the proto- 
plasm and the spermatozoon is, is not known, but the important point here is that 
the cause of the remarkable movements of the protoplasm which follow the ferti- 
