1907.] NATURAL SCIENCES OF PlIILADELl'lIIA. 353 



It. has been customary hitherto to regard such ])cculiarities of the 

 embryo as primary, and the associated peculiarities of cleavage stages 

 as secondary adaptations to these later appearing peculiarities; but 

 this puts the cart before the horse. The adaptations of cleavage are 

 adaptations to conditions pre-existing in the egg, and not adaptations 

 to conditions which appear later in the embryo or adult. Given the 

 enormous yolk of Fulgur or the small yolk of Crcpidida plana, and the 

 embryo must adapt itself to these conditions of the egg; or, in other 

 words, the earlier conditions in ontogeny stand in the relation of cause 

 to the later conditions, and not the reverse. 



When I first observed in Fulgur the great modifications in the loca- 

 tion of organ bases, which is unlike anything hitherto described, and 

 found, for example, that the cerebral ganglia and velum were located 

 in the posterior lip of the blastopore, I thought, for a time, that here 

 was a new^ pattern of germinal localization, and that the generalization 

 that homologous structures always come from homologous regions of 

 the egg had broken down. Further study has shown that this is not 

 the case, and that the great modifications in the location of embryonic 

 organs in Fulgur are not primary but secondary, while the localization 

 pattern in the early cleavages is the same as in other gasteropods. 



Hence I regard the case of Fulgur as a triumph for the method and 

 doctrine of cell-lineage. Those who see in this method only 'the 

 counting of cells,' 'mitotic book-keeping,' 'the drudgery of dull minds,' 

 have missed the whole point and significance of this method, which is 

 not to name every cleavage cell, but to determine in what areas of the 

 egg certain morphogenetic processes are located. To know that such 

 processes may be localized in the egg is valuable information, even 

 though the pattern of this localization should differ for every animal; 

 but to have discovered that through all the multifarious modifications 

 which are found in the embryos and adults of great animal classes, such 

 as the gasteropods or annelids, this same pattern runs unchanged, — 

 this is illuminating. 



Summary. 



1. The eggs of Fulgur carica are among the largest of gasteropod 

 eggs, their relatively great size being due almost exclusively to the 

 great quantity of yolk which they contain. These eggs are thirteen 

 times the diameter and about 2,000 times the volume of those of 

 Crepidula plana, with which particularly they are compared. 



2. The cleavage of the egg of Fulgur is, cell for cell, like that of 

 Crepidula up to the GG-GO-cell stage, the only difference being in the 



