ORGANIZATION AND CELL-LINEAGE OF THE ASCIDIAN EGG. 



By Edwin (1. Conklin. 



Professor of Zoology, University of Pennsylvania, 

 WITH PLATES I-XII. 



INTRODUCTION. 



A. Organization of the Egg. Recent 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. No 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 therein- lost much of its startling and 

 picturesque character, it is none the less a real and important controversy today. 

 In general the attitude of physiologists and those who deal with the processes of 

 development has ever been to jdace emphasis upon the epigenetic 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 the 

 eerm-laver theory, since the time when germ layers were considered to be the 

 earliest appearing differentiations which could be profitably compared and homolo- 

 gized. More recently it has Been shown that such differentiations appear at a 

 stage much earlier than the formation of the germ layers, ami that many of 

 the early cleavage cells of different animals show such fundamental resemblances 



l .li lUKN. A. N. S. PHILA., VOL. XIII. 



