ORGANIZATION AND CELL-LINEAGE OF ASCIADIN EGG. 109 
the weakness of the annelidan hypothesis of the origin of chordates and has adduced 
much evidence in favor of the view that the great metazoic stems run back to 
simple and minute pelagic ancestors whose common meeting place must be be found 
in still more recent times. The earliest differentations of the egg seem to me to 
favor this view. 
In conclusion, then, it seems necessary to recognize several types of cytoplas- 
mic localization. Between annelids and mollusks the similarities of localization 
extend to the bases of numerous parts and organs, thus confirming the view of 
the phylogenetic relationship of these two phyla based upon the resemblances in their 
cleavage stages and larve. Between the annelid-mollusk type of localization and 
the types found in the other phyla enumerated there are general agreements in the 
localization of the materials of the germinal layers, but few, if any, resemblances 
which extend to the bases of particular organs. The annelids do not approach 
the chordates nor the echinoderms in the earliest stages of localization any more 
closely than in their cleavage stages or later development. In all respects in which 
localizations differ in the eggs of these animals they resemble the later differences 
in their embryos. Zn short, there zs no convergence toward a common type of local- 
tization as one goes back to earlier and earlier stages in the ontogeny. 
Important results flow from this conclusion, for the doctrine that “ Ontogeny 
is a short recapitulation of Phylogeny” assumes that there is such convergence 
‘toward a common type of structure іп the early stages of development. If there be 
no such convergence the causes of the resemblances which exist between certain 
eggs, cleavage stages, embryos, larve and adults must be sought in some other 
direction. Students of the cell-lineage of annelids and mollusks have maintained 
that homologies of cleavage must be due to similarities in the protoplasmic struc- 
ture of the cleavage cells. "The same must also be said of the organization of the 
egg before cleavage begins. Similarities in the material substance of the egg and 
in the form of its localization must lie at the bottom of all later appearing simil- 
arities. And this fact, upon which all students of cell lineage have insisted, 
furnishes a possible explanation, as Morgan (1903) has recently pointed out, of 
the resemblances between the embryos of related forms. 
Speculations as to the origin and evolution of types of germinal organization are 
likely to be more interesting than valuable in the present state of our knowledge. 
Wilson (1892) first suggested that the localization of the materials of embryonic 
parts or organs in certain cleavage cells was an illustration of the principle of **pre- 
cocious segreg ation" first propounded by Lankester and afterward elaborated by 
Hyatt, in its application to paleontology, under the title of “the law of accelera- 
tion.” Lillie (1895) maintained that “it is parallel precocious segregation which 
conditions cell homologies,” and he further showed (1899) that the size and rate of 
division of individual cells in every case possesses prospective significance ; in 
short, that the cleavage forms are beautifully adapted to produce a given type of 
adult structure. Recently Wilson (1903, 1904) has expressed the view that the 
earliest differentiations and localizations of the egg, even е pU begins, are | 
examples of this same principle of “ pesce L eec 
