ARTIFICIAL PARTHENOGENESIS OF ASTEKIAS. 



65 



first polar body. Mathews ('01) claims to have been able to trace 

 the origin of the centrosome from a small granule (which divides 

 into two bodies before passing out of the germinal vesicle) within 

 the nuclear membrane, and is of the opinion that these bodies 

 migrate to the periphery of the chromatic material and become the 

 centers of the first polar division. Careful search through the 

 chromatin material in Asterias before maturation sets in shows no 

 structure that might be comparable to that described by Mathews. 

 If there are such structures they are indistinguishable from the 

 other granules of the cytoplasm. In no case has it been pos- 

 sible to identify by the usual criteria small, dark-stained bodies 

 that give the appearance of a centrosome in the cytoplasm of the 

 egg. Mathews has described the rupture of the nuclear wall 

 where the central bodies pass from the chromatin material into 

 the cytoplasm. These experiments show no evidence of such a 

 condition existing in Asterias. 



In the formation of the second polar spindle the evidences in 

 this paper agree with Tennent and Hogue ('06), and King ('06), 

 that the amphiaster is formed by the division of a monaster that 

 has arisen " de novo " on the median side of the scattered chromo- 

 somes. The second polar spindle rests tangential to the periph- 

 ery 'of the egg until it has reached its normal size and the chromo- 

 somes are pulled into the equatorial plate. The spindle then 

 rotates and migrates to the periphery of the egg and extrudes the 

 second polar body. Mead ('98) and Jordan ('08) believe they 

 have definitely observed the outer aster of the second polar 

 division to appear at the old-mid-body in the position of the old 

 equatorial plate of the first division. The second polar spindle 

 has been definitely observed in this material to be formed by the 

 division of the secondary monaster and comes to lie in a position 

 tangential to the periphery of the egg. No trace of a mid-body 

 could be found as described by Mead and Jordan after the disap- 

 pearance of the first polar spindle. Therefore, these evidences 

 show that the contentions of Mead and Jordan do not apply to 

 this material. 



Buchner has described the centrosome of the second polar 

 spindle to persist and give rise to the first cleavage amphiaster. 

 He contends that the inner centrosome of the second maturation 



