STUDIES IN ARTIFICIAL PARTHENOGENESIS. III. 



CORTICAL CHANGE AND THE INITIATION OF 



MATURATION IN THE EGG OF CUMINGIA. 1 



L. V. HEILBRUNN 



This study is a record of experiments performed during the 

 summer of 1916 at the Woods Hole Marine Biological Laboratory. 



As Morgan pointed out in 1910, the egg of the lamellibranch 

 Cumingia is very suitable for study. Like most other eggs it is 

 still immature when shed into the sea-water. Although the first 

 maturation spindle has formed, no polar bodies are thrown off 

 unless the egg is fertilized or treated with the proper reagents. 

 Doubtless some change is necessary before the egg can throw off 

 polar bodies and begin its development. 



An effort has been made to determine the nature of this 

 change. Many diverse reagents cause the egg to ma-ture. Al- 

 though all of these reagents do not occasion the same morpho- 

 logical transformations nevertheless all of them agree in having 

 one specific physical effect on the egg. All release the egg cyto- 

 plasm from the restraint of a rigid enveloping membrane. The 

 immature unfertilized egg is surrounded by a stiff vitelline 

 membrane which presses tightly in on it and effectively prevents 

 the throwing off of polar bodies. It is only when the egg is 

 released from this restraint that maturation can proceed. 



PHYSICAL MAKE-UP OF THE EGG. 



As in Arbacia, the Cumingia egg is a mass of fluid protoplasm, 

 surrounded by a rigid membrane. Only a few turns of the 

 centrifuge are sufficient to throw to opposite poles of the egg the 

 substances suspended in the cytoplasm. To one pole pass the 

 presumably lighter oil globules, to the opposite pole the heavier 

 pigment. But these suspended particles can go no farther than 

 the poles, for there they are stopped by the vitelline membrane 

 which surrounds the egg. This is a stiff structure and is easily 



1 Contribution from the Zoological Laboratory, University of Michigan, New 

 Series, no. 3. 



317 



