92 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
the median plane of the embryo is not determined by the chance path of the 
spermatozoon within the egg, but rather that both the median plane and the 
path of the spermatozoon are determined by the structure of the cytoplasm. 
Finally, in cases of normal or artificial parthenogenesis the median plane 
cannot be determined by the path of the spermatozoon. In eggs of this kind 
the establishment of bilateral symmetry must be held to be due to the structure 
of the egg itself or to environment, and whichever of these views may be accepted 
it follows that the path of the spermatozoon cannot be regarded as a general factor 
in determining the median plane of the embryo. 
These and other similar considerations lead to the view that bilateral organi- 
` gation is frequently present in the egg before it becomes visibly manifest, and 
they lend support to the hypothesis of Driesch (1896) that the eggs of all bilateral 
animals are bilaterally organized, there being a “polar bilateral direction of par- 
ticles’ in the “intimate structure of the egg." Jf this be true, the eggs, the cleav- 
age stages and the blastule of annelids and mollusks, of echinoderms and Amphi- 
oxus are as truly bilateral as they are тп the ascidians, though this bilaterality 
тау be masked бу а radial form of cleavage and by an apparently radial organt- 
zation of the egg. | 
I cannot pass over this subject without referring to the extensive work of 
Roux (1885, 1885, 1887, 1902, 1905) on the determination of the median plane in 
the frog’s egg. This work is too widely known to require more than passing 
notice. Ву means of * localized fertilization," z. e., the application of spermatozoa 
to any meridian of the egg, Roux has determined that the first cleavage plane 
passes through the entrance point of the spermatozoon and that the median plane 
of the embryo usually coincides with the first cleavage plane. Не therefore con- 
siders that the median plane is in typieal conditions, determined by the path of the 
spermatozoon. Moskowski (1902), on the other hand, holds that the first cleavage 
plane and the median plane of the embryo are determined by definite movements 
of the egg substance and not by the path of the spermatozoon. Castle (1896) 
believed that the plane of the first cleavage and the median plane of the embryo 
were determined, in the ascidians studied by him, by the place of entrance of the 
spermatozoon, the point of entrance marking the posterior pole; but since the point 
of entrance is near the vegetal pole, while the posterior pole lies near the equator, 
it is evident that the point of entrance cannot mark that pole. It is true that the 
protoplasm which gathers around the head of the sperm as soon as it enters the egg 
moves with the sperm to the posterior pole and there remains permanently, but the 
location of this protoplasm at this pole is evidently due to something other than the 
point of entrance of the spermatozoon. There is no question whatever that, in the 
ascidians, the path of the sperm within the egg coincides with the plane of the 
first cleavage and with the median plane of the embryo, but there is evidence, as 
I have shown, that this path is itself determined by the structure of the egg. 
