36 THE ASTER IN ARTIFICIAL PARTHENOGENESIS 



ters connects in some way with the monaster, thus forming the amphi- 

 aster which initiates segmentation. The weakness in this interpre- 

 tation is the lack of conclusive evidence for the union of the originally 

 independent asters. Neither Wilson nor Hindle ever observed such 

 a phenomenon. All my observations also indicate that the amphi- 

 aster in parthenogenetic eggs arises from a previous single aster just 

 as it does in normally fertilized eggs. 



My studies were mainly confined to the egg of the sand-dollar. 

 In its behavior to parthenogenetic agents^ the egg is almost identical 

 with that of the sea-urchin which Herlant studied. The absence of 

 pigment and the highly translucent nature of its protoplasm makes 

 the sand-dollar egg an ideal object for observational study. 



The mature eggs, normally shed by the female, are placed in buty- 

 ric acid (2 cc. 1/10 n in 50 cc. of sea water) for 35 seconds. During 

 this treatment the eggs distinctly round up. They are then returned 

 to sea water where, within a few minutes, the fertilization membrane 

 lifts off. After 20 minutes the eggs are placed in hypertonic sea 

 water (5 cc. 2.5 m NaCl in 50 cc. sea water). The eggs shrink slightly 

 in this solution. After 20 minutes the eggs are transfered to a large 

 quantity of normal sea water and the sea water is changed several 

 times to free the eggs from any further action of the hypertonic 

 solution. 



Up to this time no change whatever is to be seen in the cytoplasm 

 or in the nucleus. While in the hypertonic solution the cytoplasm 

 appears more granular and opaque than that of an untreated mature 

 egg. However, on the return of the treated eggs to sea water the 

 cytoplasm reverts to its former appearance and to the eye the eggs 

 differ in no respect whatever from unfertilized eggs except for the 

 presence of a fertilization membrane. 



It is not until the treated eggs have stood in sea water for several 

 minutes that any cytoplasmic change is to be observed. The first 

 sign of a change consists in the appearance of faintly defined vacuoles 

 about the center of the egg. Within a few minutes they coalesce 

 to form a central clear area of about one-tenth the diameter of the 



^Just, E. E., The fertilization reaction in Echinarachnius parma. III. The 

 nature of the activation of the egg by butyric acid. Biol. Bull., 1919, xxxvi, 39. 



