110 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



Although this principle is carefully stated so as not to directly affirm that the 

 organization of the egg is the result of the organization of the adult, or that the 

 adaptations of the early development have arisen secondarily after the adult struc- 

 ture was estafTHshed, these ideas are nevertheless plainly implied. The early 

 appearance of differentiations is usually explained as a " throwing hack of adult 

 characters upon the egg." The whole life cycle is viewed from the standpoint of 

 the adult ; the embryo and germ exist for the purpose of producing a certain end ; 

 the adult is primary, the germ secondary. But do not all such ideas put the cart 

 before the horse ? What is the evidence that any inherited modification of an adult 

 structure can arise without an antecedent modification of the germ ? We know that 

 the adult is moulded upon the egg, that specific modifications of the germ do, in 

 some cases, produce specific modifications of the adult, but the converse proposition 

 is certainly not established. "Precocious segregation" represents the backward 

 rather than the forward look; it is a teleological rather than a causal explanation. 



As there can be no transmission of heritable qualities from one generation to 

 another except through the germ cells, so there can be no evolution of adult forms 

 except through the evolution of the germ cells. Any inherited modification of a 

 species implies some modification of the germ cells of the species. Even " accel- 

 eration" or " precocity" must be due to a modification of the germ in its earliest 

 stages, a modification of some unknown sort which hastens differentiation. 



It cannot be maintained that all those animals in which differentiations and 

 localizations are present in the unsegmented egg are, for that reason, debarred 

 from any further evolution, but if this be not true then it must follow that the 

 type of egg organization must undergo modifications during the course of evolution, 

 and granted this we have no need of the principle of " parallel precocious segre- 

 gation " for explaining any of the homologies of the earl}' development. If the 

 resemblances between annelids and mollusks are not due primarily to the similari- 

 ties in the adults or larva? or cleavage stages, but to phylogenetic similarities in the 

 organization of the unsegmented egg, we have in this initial resemblance a sufficient 

 explanation of all later resemblances, whereas if we reverse this procedure and 

 hold that the similarities of the adults or larva? are the causes of the likenesses in 

 the earlier stages we must of necessity resort to some such teleological principle 

 as "precocious segregation" for an explanation. 



In view of the fact that there are such definite types of differentiation and 

 localization in the eggs of many animals and that the causes which lead to the 

 evolution of animals must operate through modifications of this organization, the 

 character and manner of such modification become problems of the first importance. 

 If the nuclear inheritance theory is true, such modifications must in the first inst- 

 ance affect the chromosomes ; but how and in what respect is wholly unknown. In 

 the case of the cytoplasm it is evident that such modifications may concern the char- 

 acter, or quality, of the differentiations and the place and manner of their locali- 

 zation. Modification of any of these might be expected to produce modifications 

 in the resulting animal. 



