Introductory 13 



following statement may be made concerning the differen- 

 tiation of the uninucleated eggs. (1) Organs are never 

 formed, but only -such structural elements as may occur 

 in single cells of the trochophore. (^) Organs may, how- 

 cvcr, be simulated by the aggregation of the characteristic 

 matter of the organ, for instance in the case of the yellow 

 endoplasm, which simulates the gut of the trochophore, or 

 the row of large vacuoles *ituatcd near the upper margin 

 of the yellow endoplasm which simulates the row of vacuoles 

 of the prototroch. ('$) Structural elements appear in the 

 same order of time as in the trochophore. (4) The distri- 

 bution of the structural elements tends to resemble that of 

 the trochophore. (5) The yellow endoplasm (yolk?) is used 

 up, apparently for the maintenance of the metabolism in the 

 ciliated unsegmented eggs precisely as in the larva." ' The 

 theoretical bearings of the observations are indicated by the 

 ollowing: "The possibility of a considerable amount of 

 bryonal differentiation without either nuclear or cyto- 

 asmic division may be considered established. This in it- 

 self is an important fact, for it disposes effectually of all 

 theories of development that make the process of cell-division 

 e primary factor of embryonal differentiation, whether 

 the form of Weismamfs qualitative nuclear division, or 

 ert wig's cellular interaction theory. Further, the phe- 

 nienon establishes firmly, as I pointed out in li)01, the 

 iew that the role of cell-division in development is prima- 

 ily a process of localization. 11 



Lillie presents his still broader interpretation in an ex- 

 ingly interesting section headed "Properties of the 

 hole ( Principle of I'm'ty)." From this I quote somewhat 

 ore at length than is essential for our immediate purpose 

 f gaining a bird's-eye view of the field \ve are entering, since 

 ater we shall want to examine several of the items more 

 closely. "The traditional view, held by main embryologists 

 at the present day, is that the physiological unity arises in 



