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ORGANIZATION AND CELL-LINEAGE OF THE ASCIDIAN EGG. 
By Epwin С. CONKLIN. 
Professor of Zoology, University of Pennsylvania, 
WITH PLATES I-XII. 
INTRODUCTION. 
A. ORGANIZATION оғ THE Ecc.—hRecent years have witnessed a revival of 
the ancient controversy as to the nature and contents of the germ cells. On 
the one hand are those who with Weismann maintain that the egg must contain 
the elements or determinants of very many structures which will appear in the 
course of development; on the other hand are ranged the modern epigenesists 
who find in the egg cell only complex chemical substances which have the, capacity 
under certain outer conditions of undergoing regular transformations into other 
substances which incidentally have peculiar forms, just as crystals have. 
But while this modern controversy recalls the ancient one between the 
adherents of evolution and those of epigenesis, it does so chiefly because it proceeds 
from the same temper of mind, and not because anyone today is ready to defend 
the views of either the evolutionists or the epigenesists of a century ago. Хо one 
now expects to find in the egg or sperm a predelineated germ with all adult parts 
present in miniature, neither can anyone now maintain that the egg is composed 
of unorganized and non-living material. Everyone now admits that the truth is 
somewhere between these two extremes; the real problem is how much or how 
little of organization is present, and not whether the germ is organized at all. 
Though the controversy as to evolution and epigenesis has thus been nar- 
rowed within relatively small limits, and has thereby lost much of its startling and 
picturesque character, it is none the less а real and important controversy today. 
In general the attitude of physiologists and those who deal with the processes of 
. development has ever been to place emphasis upon the epigenetie character of 
development and the extremely simple structure of the germ; whereas those who 
are concerned chiefly with organic structures are prone to seek for antecedent 
structures in earlier and earlier stages of development and so finally in the 
unsegmented egg itself. 
It is not many years since all embryological studies were dominated by г ihe 
germ-layer theory, since the time when germ layers were considered to be the 
earliest appearing differentiations which could be profitably compared and homolo- 
gized. Моге recently it has been shown that such differentiations appear at a 
stage much earlier than the formation of the germ layers, and that many of 
the early cleavage cells of different animals show such fundamental resemblances 
1% JOURN. А. М. 8. PHILA., VOL. XIII. 
