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. In both 

 the ascidian 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. In 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. The 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 egg 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. In 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. In 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 animalpole 

 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- 



