504 CELL DIVISION IN EGGS OF CREPIDULA. 



the sequence and origin of differentiations and the extrinsic and intrinsic factors 

 by which they may be modified. Much has already been accomplished in 

 tracing the sequence of morphological differentiations to earlier and earlier stages 

 of development, but relatively little has been learned as to the nature and causes 

 of differentiation itself. By the observation of purely normal processes one 

 sooner or later comes to a place beyond which he can make little if any progress 

 in the study of such problems, but the history of biology in the last twenty-five 

 years has shown that much may be learned by the comparison of normal and 

 abnormal processes, and especially by the combination of observations of normal 

 processes with experiments which may be varied indefinitely. 

 ' In two previous publications (1897, 1902) I described in detail the normal 

 development of the gasteropod, Crepidula, particularly as regards the phenomena 

 of cell-lineage and of nuclear and cell division. The present work comprises 

 the results of a series of about 260 separate experiments, extended over a period 

 of more than ten years, on the eggs of Crepidula plana, the species which was 

 principally used in my previous work. The eggs of this animal afford unusually 

 fine material for the study of the normal processes of development, but they are 

 less favorable for experimental work and for the following reasons: (1) Fertili- 

 zation takes place within the oviduct, and eggs capable of development cannot 

 be obtained until after they are normally fertilized and laid. (2) The laid eggs 

 are inclosed in capsules within which they undergo the whole of their embryonic 

 development, and when removed from these capsules they do not continue to 

 develop normally for more than three or four days. (3) The rate of develop- 

 ment is so slow, about one month being required to reach the stage of the free- 

 swimming larva, that it is difficult to rear larvae from eggs which have been 

 subjected to experiment. But while these conditions make this material un- 

 favorable for experimental studies on fertilization or the development of larval 

 or adult organs, they do not seriously interfere with experimental studies on 

 cleavage and the early differentiations of the egg. On the contrary the eggs of 

 Crepidula are peculiarly favorable for such studies. In no other egg with which 

 I am acquainted is it possible to study the details of cell organization so satis- 

 factorily as in this one. The eggs may be obtained in large numbers and may be 

 stained and mounted entire so as to show minute' details of cell polarity, the 

 structure and division of nuclei, centrosomes and cytoplasm, and the relation of 

 cell growth and division to differentiation. In such a case the advantage of the 

 study of whole eggs over that of sections is very great; one can see at a glance 

 the relative positions of all cell constituents, and of all the cells in the cleaving 

 egg, and one may readily study and compare these minute details in hundreds of 

 eggs, whereas this could be done in the case of sections only by the most laborious 

 methods and then only in a relatively small number of eggs. Of course sections 

 have also been used in cases where it seemed necessary in order to answer 

 questions which were left more or less uncertain by the evidence derivable from 

 whole preparations. 



