ORGANIZATION AM) CELL-LINEAGE OF ASCID1AX EGG. 91 



plasm of the unsegmented egg, one on each side of the polar bodies. Blochmann 

 (1882) observed that these granules were ultimately localized during cleavage 

 in the right and left " Urvelarzellen." They therefore mark out a bilateral organ- 

 ization of the unsegmented egg, although the cleavage up to the time of the forma- 

 tion of the Urvelarzellen" is typically spiral and radially symmetrical. In other 

 gasteropod eggs, where these granules are lacking, not a trace of bilateral organiza- 

 tion is visible before the formation of the mesentoblast cell ; yet it can scarcely be 

 supposed that the eggs of these gasteropods are so unlike those of Neritina as to be 

 actually radially symmetrical as they appear to be. Rather it seems probable that 

 the bilateral organization which appears in this one respect in the Neritina egg is 

 characteristic of other gasteropod eggs also, though it does not usually become 

 apparent until a later stage. 



Crampton (1894) discovered that the cleavage of the egg in sinistral snails is 

 reversed as compared with that of dextral forms. I have shown elsewhere (1903) 

 that the inverse symmetry of sinistral snails is traceable to the inverse organization 

 of the unsegmented 'egg. Of this fact there can be no doubt, though it is not yet 

 certain how this inverse organization may have been produced. But an inverse 

 organization of the egg, such as would produce inverse symmetry of the embryo 

 and adult, implies of necessity a bilateral organization to begin with ; it must be, 

 therefore, that the eggs of these gasteropods are bilateral, though this fact is not 

 directly evident. 



In the aseidian egg the first appearance of bilaterality which I have been able 

 to detect occurs soon after fertilization when the sperm nucleus moves toward one 

 side of the egg which later becomes the posterior pole. One might, therefore, lie 

 inclined to consider that in this case the egg before fertilization was radially symme- 

 trical, and that the chance movement of the sperm into one meridian determined 

 the median plane of the embryo, were it not for the fact that all the movements of 

 the sperm within the egg seem to be directed by the organization of the cytoplasm. 

 The sperm always enters the egg near the vegetal pole, but the fact that the point 

 of entrance is nearer that pole in some instances than in others shows that that 

 point is not a fixed and constant one. After the sperm has penetrated the peri- 

 pheral layer of protoplasm, and has turned so that its centrosome is directed for- 

 ward in its movement through the egg it moves up to the equator of the egg in a 

 path nearly parallel with the surface. Arrived at the equator, the upward move- 

 ment ceases and the sperm nucleus and centrosome. after meeting the egg nucleus, 

 turn in toward the center of the egg. These movements are of such a con- 

 stant character that they cannot be the result of chance; they must be directed 

 and probably by the cytoplasm of the egg. Furthermore, it seems probable from the 

 evidence of such cases as figures 81 and 85 that the sperm nucleus does not 

 always take the shortest path to the equator as it should do if the egg were 

 radially symmetrical and the median plane were really determined by the path 

 <>f the spermatozoon . On the other hand, it sometimes appare)itly takes the longest 

 path as if it must needs move in a certain meridian. This seems to indicate that 



