BILATERALITY OF THE PIGEON's EGG 305 



the fertilized eggs of the ascidians. It will be of interest then 

 to consider briefly the evidences and indications of bilateral organ- 

 ization that have been found in vertebrate eggs. 



Myxinoids. The ovarian eggs of Bdellostoma have been de- 

 scribed as being bilaterally symmetrical, their shape correspond- 

 ing somewhat to that of many insect eggs. Dean ('99) denied 

 this for ovarian or newly laid eggs, but it must be remembered 

 that the question is complicated here, since the eggs are subjected 

 to a great and rapid change in pressure when they are collected. 

 The possibility exists that the antero-posterior axis of Bdell- 

 ostoma may arise in the ovary. 



Selachians. The selachian egg presents many striking resem- 

 blances to that of the bird, as is to be expected, since these two, 

 together with the reptilian egg are the most extreme types of 

 meroblastic ova. The segmental disc in the selachians, as in the 

 birds, is surrounded bj^ a periblast; in the latter Riickert ('92) 

 found evidences of bilateral symmetry while the egg was still 

 in the ovary. He described in Torpedo an extension of the 

 marginal periblast surrounding the blastodisc and projecting down 

 into the yolk mass. This 'Mantel' showed considerable vari- 

 ation in form, but it was found to be constantly deeper and more 

 sharply defined at one side of the disc than the other. In some 

 eggs this structure could be traced from late stages of ovarian 

 eggs, through the fertilization and cleavage stages, and rarely 

 to the time of gastrulation when it appeared as if the deeper 

 side of the 'Mantel' were posterior. Riickert pointed out the 

 significance of this yolk configuration, if it were differentiated 

 along the antero-posterior axis, but he did not consider his evi- 

 dence as adequate to warrant a definite conclusion. I may say 

 that I have found similiar conditions in the eggs of Raja ocellata. 

 The variation in these axial relations is to be accounted for as 

 follows : The relations are only expressions of the bilateral organ- 

 ization, not the organization itself; and so the expression varies 

 in different eggs, but is relatively constant for the eggs of a given 

 bird. The relations may be modified by other factors in develop- 

 ment but the bilateral organization is nevertheless present; in 



