110 ORGANIZATION AND CELL-LINEAGE ОЕ 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 established, these ideas are nevertheless plainly implied. The early 
appearance of differentiations is usually explained as a “throwing back of adult 
characters upon the egg." Тһе 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 adultis moulded upon the ере, 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 ofthe germ cells. Any inherited modification of a 
species implies some modification of the germ cells of the species. Even “ассеі!- 
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 early development. If the 
resemblances between annelids and mollusks are not due primarily to the similari- 
ties in the adults or larve 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 1агуз 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, 
