THE PHYSICAL BASIS OF LIFE 



39 



ing zone (Fig. 15). In the asoiclian, too, the striking visi- 

 ble pattern seen at the beginning of cleavage does not at 

 first exist but is swiftly built under the eye of the observer 

 by streaming movements of the ooplasm that take place 

 immediately upon entrance of the sperm into the egg 

 (Fig. 17). In cases like these, where the localizing opera- 



D ^-- -^ E -^^JZ.,^^ F 



FIG. 18 

 liOcalizing activities in the eggs of an aseidian Cynthia, showing early localization 

 of the regions of the future embryo, as indicated by differently shaded areas 

 (from CoNKLix). The bilaterality of the future embryo is clearly indicated in 

 D (side view) and E (rear view) ; and the first cleavage-plane in F marks ex- 

 actly the median jdane. 



tions may readily be followed by the eye, the egg offers an 

 impressive spectacle when Inisily engaged at its work of 

 blocking out the embryo, without visible tools or model, 

 but with an uncanny air of delil)erate purpose and mas- 

 tery of technique that any human artist might envy. 



Beyond a doul)t the movements and regroupings of 

 material which give rise to the visible pattern are expres- 



