ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 91 
plasm of the unsegmented egg, опе 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 есе, 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 Merz/zna 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 Merztzna 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. ОҒ this facet 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 ascidian 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, be 
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 
of the spermatozoon. On the other hand, 1t sometimes apparently takes the longest 
path as if it must needs move тп а certain meridian. This seems to indicate that 
