306 GEORGE W. BARTELMEZ 



other words, these relations simply give evidence that such an 

 organization exists. 



It may be then that the selachian and bird's eggs agree not 

 only in that the axis of bilaterality appears in the ovary but also 

 in that the first clear evidence of the bilateral organization of 

 the blastodisc is found in the marginal periblast. This is the 

 only instance that has come to my attention in which definite 

 morphological evidence was found indicating the ovarian origin 

 of the axis of bilateral symmetry in a vertebrate. 



Teleosts. Several authors have noted that the blastodisc of 

 teleost eggs is thicker on one side than on the other; Oellacher's 

 (72) figure 17 indicates that this difference exists before cleavage 

 begins, i.e., that the thickening of the blastodisc after the laying 

 of the egg is more rapid on one side. Agassiz and Whitman, 

 ('85) and Kowalewsky ('86) consider this an antero-posterior 

 differentiation, the thicker being the posterior side, though nothing 

 is said of continuous observations to confirm this. It is quite 

 possible that here as in the Amphibia fertilization initiates the 

 appearance of the axis of bilaterality. 



Amphibians. Many workers, notably Roux, Brachet ('05) and 

 Schultze ('99) , have shown that the amphibian egg is bilaterally 

 organized previous to cleavage. This organization expresses itself 

 in a definite movement of the superficial pigment granules so that 

 a gray crescent appears at the posterior side of the egg, the center 

 of the crescent marking the position of the blastopore in later 

 stages. It seems unlikely, from what is known of other eggs, 

 that the point of entrance of the spermatozoon determines the 

 axis of bilaterality, as Roux has maintained ; bilaterahty manifests 

 itself, independently, after the stimulus of fertihzation has been 

 received. While the copulation path may determine the plane 

 of the first cleavage, both may be quite independent of the axis 

 of symmetry, as Brachet's work has clearly shown. The arrange- 

 ment of^the different kinds Of yolk seems to be radially symmetri- 

 cal, and how far the bilateral configurations of pigment granules 

 described by Van Bambeke ('76) in the toads' egg are related to 

 the bilaterality of the embryo and how far they are due to the 

 mitotic forces, which here seem to be independent, remains to 



