44 LILLIE. [Vol. X. 



body. In other words, the blastomeres of segmentation stages 

 have been shown to be the elements of a mosaic. This 

 is a fact which no amount of argument or experiment can 

 remove. 



It remains for us to find out how these parts are made, how 

 put together. Inasmuch as we have not as yet the entrance 

 to the room of the raw materials in the workshop, we must 

 study the products, the blastomeres, as they are being formed 

 and after their formation. We must stop the process at each 

 stage and fix it for the most careful and leisurely of examina- 

 tions ; when we have studied every stage of a division known 

 to be differential, and have analyzed all the observable factors, 

 the relation of the chromosomes in the two resulting cells and in 

 each phase of division ; of the asters ; of the general cytoplasm ; 

 and have found out where the earliest indications of the cleavage 

 manifest themselves ; then, if we remember that we are observ- 

 ing but the finishing touches to the most elaborate and delicate 

 of mechanisms, it may be that we can argue with some sound- 

 ness on the philosophy of differentiation. 



Conklin has cited some suggestive facts in this connection 

 (No. 41, p. 33) : in Crepidula however unequal the division of 

 the cytoplasm may be, there is always an equal division of the 

 asters and of the chromatin. " Yet in those very stages in 

 which the nuclei and asters are equal in size, the lobing of the 

 cytoplasm may show beyond doubt that the division of the 

 cell-body is to be very unequal." However, before the cells 

 have separated, the asters have become proportional in size 

 to the surrounding cytoplasm ; soon after the separation, the 

 nuclei become proportional in size to the cell-body. In Unio 

 I have observed that the resting nuclei become proportional in 

 size to the cytoplasm, though in the different cells they contain 

 originally the same amount of chromatin. Boveri (No. 37^) 

 has observed that polar bodies in Ascaris megalocephala will 

 act in the same way as the female pronucleus when brought 

 under similar conditions. How instructive is his account of 

 the continuity of the form of the chromosomes in the germ- 

 cells alone in Ascaris, while in the somatic cells this form is 

 lost from their origin ! (No. 37.) Observations of this sort 



