Introductory 13 



following statement may bo made concerning the differen- 

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

 formed, but only such structural elements as may occur 

 in single cells of the trocho])hore. (^) Organs may, how- 

 ever, 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 situated near the upper margin 

 of the yellow endoplasm which simulates the row of vacuoles 

 of the prototroch. (3) 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 

 following: "The possibihty of a considerable amount of 

 embryonal differentiation without either nuclear or cyto- 

 plasmic division may be considered established. This in it- 

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

 theones of development that make the process of cell-division 

 the primary factor of embryonal differentiation, whether 

 in the form of Weismann's qualitative nuclear division, or 

 Hertwig's cellular interaction theory. Further, the phe- 

 nomenon establishes firmly, as I pointed out in 1901, the 

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

 rily a process of localization.^^ 



Lillie presents his still broader interpretation in an ex- 

 ceedingly interesting section headed "Properties of the 

 Whole (Principle of Unity)." From this I quote somewhat 

 more at length than is essential for our immediate purpose 

 of gaining a bird's-eye view of the field we are entering, since 

 later w^e shall want to examine several of the Items more 

 closely. "The traditional view, held by many embryologists 

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



