No. 3.] THE CELL-LINEAGE OF AEREIS. 363 



transparent and of a convenient size ; the differentiation of the 

 blastomeres appears at a very early period ; and from the begin- 

 ning of its development the embryo possesses a number of pecu- 

 liar features, by means of which it may at any stage be perfectly 

 oriented, almost at a glance.^ Furthermore, the eggs may eas- 

 ily be fixed and stained by a simple method, which shows with 

 great clearness both the cell-outlines and the nuclear figures. 

 This combination of favorable characteristics renders it possible 

 to trace out the cellular genesis of various parts of the body, 

 step by step from the beginning of development, with a com- 

 pleteness and precision that is rarely attainable. The results 

 show, as I am convinced, that much is to be gained by consid- 

 ering the embryological development of animals from a point of 

 view differing in some respects from that usually adopted, and 

 this point of view I wish to make clear before proceeding to a 

 detailed description of the facts to be set forth. 



For many years (more especially since the publication of 

 Haeckel's celebrated papers on the gastrsea theory) embryologi- 

 cal research has been dominated by certain general conceptions, 

 usually designated as the "germ-layer theory," a term which 

 need not be more precisely defined here. The germ-layer theory 

 forms, in fact, the foundation on which the entire science of 

 comparative embryology is built. And yet it does not require a 

 very extensive search among the embryological writings of the 

 last decade to discover that a surprising divergence of opinion 

 exists among the best authorities in regard to some of the 

 most fundamental propositions of this theory. This divergence 

 need not here be reviewed in extenso, but I may be allowed 

 to illustrate its character by quoting somewhat fully from 

 two eminent embryologists. Ten years ago, Balfour wrote as 

 follows :^ — 



" Since there are some Metozoa with only two germinal lay- 

 ers, and other Metozoa with three, and since . . . the third layer 

 or mesoblast can only be regarded as a derivative of one or both 

 the primary layers, it is clear that a complete homology between 

 the two primary germinal layers does not exist. 



1 Dr. E. A. Andrews first called my attention to the favorable character of the 

 Nereis eggs and the ease with which they may be procured in large numbers, and I 

 am glad to acknowledge my great indebtedness to him. 



2 Comparative Embryology, Vol. II, 1881. 



