RELATING TO THE ASYMMETRY OF THE GASTROPODA. 143 



These Last considerations lead us to the changes of shape undergone 

 by the embryos in early stages. Even before the Gastropod egg is 

 affected by cleavage, and while it is undergoing this process, the animal 

 and vegetative poles may be distinguished. The blastopore at first 

 corresponds to the vegetative pole, but, as it lengthens, it encroaches 

 upon the future ventral surface, while the animal pole appears to lie 

 on the dorsal surface. The part of the ectoderm that forms at the 

 animal pole seems to shift later to the anterior end of the embryo 

 around which the velum is developed. The axis which passes through 

 the animal and vegetative poles of the early stages does not, there- 

 fore, in the Gastropoda, correspond, as might be supposed, to that 

 passing through the apical plate and the anus of the larva, but lies 

 more or less at an angle to the latter. It has already been shown 

 that the definitive axes are laid down at an early stage in the embryo 

 (p. 107). The identification of these axes is by no means easy, especi- 

 ally as the shape of the larva undergoes a certain amount of modifi- 

 cation according to the quantity of yolk deposited in the egg. On 

 this account, Fol's statement that the shell-gland appears at the 

 animal pole requires further investigation. It is a striking fact, 

 however, that in the Cephalopoda it actually has such a position, 

 a fact which will be discussed later.* The shell-gland, as is well 

 known, lies dorsally on the embryo, whereas the pedal prominence 

 arises on the ventral side between the mouth and the anus. In their 

 most primitive condition the embryos, or larvae of the Gastropoda, 

 are quite symmetrical ; only later does the body become asymmetrical 

 through displacement of the internal and external organs. 



Considerations relating to the asymmetry of the Gastropoda. 



The development of the body during its ontogeny follows the 

 course which we are inclined to believe was taken by the Gastropoda 

 phylogenetically in attaining their present asymmetrical condition. 

 There can be no doubt that the Gastropoda are derived from symmetrical 

 forms, for we find the other members of the Molluscan phylum, which 

 had the same ancestors as the Gastropoda, symmetrically developed. 

 This is confirmed by ontogeny, for the symmetrical form is long retained 

 in the embryo although it is eventually lost in consequence of the 

 unequal growth of the various regions of the bod}'. It is especially 



* [For a review of the facts relating to the shifting of the larval axes see 

 Conklin (No. IV.) and Lillie (App. to Literature on Lamellibranchia, 

 No. III).— Ed.] 



