Further Examination of the Cell-Thcorij 219 



This statement about the movements of the germ-band has 

 reference to the fact that the actively developing part of 

 the Qgg makes a series of remarkable journeys, as one 

 might say, within and upon tlie rest of the egg, which con- 

 sists mostly of yolk. In other words the topogra})hv and 

 orientation of the very young insect are so far and so firmly 

 established before cell multiplication begins that the details 

 of cell splitting and cell movement and arrangement have 

 seemingly little or no influence in determining these rela- 

 tions, but on the contrary, though going on for a time 

 quite independently of such relations, are finallv brought 

 into conformity with them. 



In another group of animals, those to which "pill bugs" 

 and "sow bugs" belong, the eggs, like those of insects, con- 

 tain a great quantity of yolk but the proto})lasm is sharply 

 separated into two portions, the one superficial and non- 

 nucleated for a time, the other deeply imbedded in the yolk 

 and nucleated. Only the centrally situated nucleated por- 

 tion undergoes division at first. This division is so thor- 

 oughgoing that the resulting cells, the blastomeres, become 

 widely separated from one another and scattered through 

 the yolky part of the (tgg. Later these scattered blasto- 

 meres migrate into the surface patch of })rotoplasm and 

 uniting with it form the blastoderm, the forerunner of the 

 embryo proper. Investigating this mode of development, 

 J. P. McMurrich emphasizes the fact that all details 

 of cell-formation and migration and final arrangement have 

 reference to structural peculiarities some of which are pres- 

 ent before cell multiplication begins, while others pertain 

 onl}^ to the later embryo. Both the direction taken by tiie 

 spindle of the dividing nuclei, and the aggregation of the 

 blastomeres in the surface layer of protoplasm are, says 

 McMurrich, "simply ])recocious y)reparations for a difl'eren- 

 tiation which will later become pronounced; they refer to the 

 final form of the embryo, and are instances of Sachs' law 



