EXPERIMENTS ON THE LOCALIZATION OF DEVEL- 

 OPMENTAL FACTORS IN THE NEMER- 

 TINE EGG. 



CHARLES ZELENY. 



CONTENTS. 



1. Introduction 293 



2. Method 294 



3. Normal Development 205 



4. Experimental Results 298 



I. Unfertilized Egg 299 



II. Fertilization to Complete Separation of First Polar Body . . . 302 



III. First Polar Body to Complete Separation of Second Polar Body 302 



IV. Second Polar Body to Beginning of Lateral Elongation of Egg 306 

 V. Elongated Egg to Completion of First Cleavage 308 



VI. Two-Cell Stage 309 



VII. Four-Cell Stage 313 



VIII. Eight-Cell Stage 315 



IX. Sixteen-Cell Stage 316 



X. Blastula 320 



5. Summary 2>22 



6. General Discussion 323 



The general problem of the localization of developmental fac- 

 tors within the egg has received an important addition in a recent 

 paper by Professor E. B. Wilson, giving strong experimental evi- 

 dence of a progressive character in the localization of materials 

 in the egg of Cerebratuhis lacteus.^ 



The present paper is a description of a similar series of experi- 

 ments on the Mediterranean species, Cerebratuhis marginatus, 

 carried on at Naples during April and May, 1903, at the sug- 



lExperiments on clevage and localization in the nemertine-egg. Archiv. f. 

 Entw. der Organismen. Bd. 16 Heft 3, 1903, pp. 411-460. 



