54 HARRIETT M. ALLYN. 



induced by some physical factor, such as an increase in the 

 permeability of the egg membrane or surface caused by a change 

 in the osmotic pressure of the surrounding medium. There is 

 very little evidence for this so far, however. 



2. Problem of Maturation. In the artificial parthenogenesis 

 of Chcstopterus development may proceed whether one or both 

 polar bodies have been extruded, or both have been retained 

 within the egg. This phenomenon has been noted in other forms. 

 Lefevre ('07) found it to be so for Thalassema mellita, Delage 

 ('02) for Asterias, Scott ('06) for Amphitrite, Kostanecki ('n) 

 for Mactra, Treadwell ('02) for Podarke, and Morgan ('oo) and 

 Lillie ('06) for Chcetopterus. 



The cytological questions of spindles and chromosomes have 

 not been taken up in many cases. Lefevre ('07) found in 

 Thalassema that when only the first polar body was cast out of the 

 egg, the second polar spindle was to be seen deep within the 

 cytoplasm, as a result of which two nuclei might be formed in 

 the egg and fuse to form the cleavage nucleus. He was unable 

 to get an accurate chromosome count but was able to ascertain 

 that more than the normal reduced number appeared in later 

 stages. When no polar body was thrown off he found in most 

 cases only one maturation division and this took place within 

 the egg, with subsequent union of nuclei. These observations 

 are similar to certain phenomena which I have seen in Chcetop- 

 terus. In this egg also the second polar spindle might often be 

 seen lying deep within the egg, and forming tw r o nuclei which 

 united. Counts of chromosomes in later stages indicated that 

 at least an approximation of the normal 2n number was present. 

 Similarly when no polar body was thrown out two nuclei were 

 formed which united in certain cases. 



In Amphitrite Scott ('06) found quite a different case, appar- 

 ently. Here the chromatin of the first or second polar body 

 might be thrown off in a mass in the cytoplasm. In Asterias 

 Delage ('01) observed 18 chromosomes, the zn number, when 

 but one polar body had been thrown off. In Chcetopterus Morgan 

 ('oo) found two nuclei when the second polar body had not been 

 extruded. In Mactra Kostanecki ('n) observed that whether 

 one, two, or no polar bodies had been thrown off, cleavage of 



