ORGANIZATION AND < KLL-LINEAGE OF ASCIADIN EG<;. 109 



the weakness dl' ! he 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 placemust lie be found 



in still more recent times. The earliest dift'erentations 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 larvae. Between the annelid-mollusk type of localization and 

 tin' 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. In short, there is no convergence toward a co?n)non type of local- 

 ization 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 in the early stages of development. If there be 

 no such convergence the causes of the resemblances which exist between certain 

 eggs, cleavage stages, embryos, larvas 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 tyj3es 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 segregation" first propounded by Lankester and afterward elaborated by 

 Hyatt, in its application to palaeontology, 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 pi'ospeetive 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 before cleavage begins, are 

 examples of this same principle of "precocious segregation." 



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