92 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



the median plane of tlie embryo is not determined by tlie 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- 

 zation is frequently present in the egg before it becomes visibly manifest, and 

 they lend support to the hypothesis of Driesch (189G) 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." // this be true, the eggs, the cleav- 

 age stages and the blastulce of annelids and mollusks, of echinoderms and Amphi- 

 oxus are as truly bilateral as they are in the ascidians, thohgh this bilaterality 

 may be masked by a radial form of cleavage and by an apparently radial organi- 

 zation of the egg. 



I cannot pass over this subject without referring to the extensive work of 

 Roux (1883, 1885, 1887, 1902, 1903) on the determination of the median plane in 

 the frog's egg. This work is too widely known to require more than passing 

 notice. By means of " localized fertilization," i. 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. He therefore con- 

 siders that the median plane is in typical 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 ; I mt 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 tlie 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 e^'i. 



