38 THE PHYSICAL BASIS OF LIFE 



and a smaller lower clear zone wliich produces the larval 

 mesencliynie. In the mollusk DcntaUum the egg likewise 

 shows three zones which have a similar (thongli not pre- 

 cisely identical) relation to the formation of the larval 

 body. Experimental operations on the egg have proved 

 that these areas or "germ-forming regions" have a defi- 

 nite specification or predetermination which can be al- 

 tered only in minor degree by experimental methods. In 

 the sea-urchin, for example, fertilized fragments of the 

 egg do not gastrulate unless at least a portion of the red 

 zone be present; and in Deutaliiim removal of the lower 

 white area causes not merely the production of a smaller 

 larva but one that shows very definite and constant struc- 

 tural defects (Fig. 17). In the ascidian StijeJa partita the 

 egg, before cleavage begins, shows a still more specific 

 prelocalization, the ooplasmic pattern already showing 

 very clearly bilateral symmetry and antero-posterior dif- 

 ferentiation, and in some degree a prelocalization of the 

 materials for not less than five important structures of 

 the future larva (Fig. 18)."'' '' 



All this, evidently, demonstrates the fact of germinal 

 prelocalization and on its face seems to sustain the theory 

 of the embryo in the rough ; but a more critical examina- 

 tion dispels this latter impression. For observation and 

 experiment alike indicate that prelocalization in the 

 ooplasm is a secondary product of an earlier development 

 in the egg ; the rough model, in other words, is blocked out 

 in the ooplasm by an epigenetic process. In Boveri's sea- 

 urchins, the horizontal zones do not appear — and, so far 

 as we can determine, do not exist — before the polar bodies 

 have been extruded from the egg. Only after they have 

 been formed does the red-pigmented cortical ooplasm, at 

 first extending around the whole periphery of the egg, 

 stream toward the equator to form the entoderm-form- 



