234 LILLIE. [Vol. XVII. 



distinct ' male ' centrosomes, spindle, and radiations. The 

 following stages afford a further confirmation of the view that 

 this disintegration is a part of the normal history of these 

 structures, for a little later there is no trace of asters or 

 spindles in this egg." Concerning the cleavage centers, Child 

 says : " I am inclined to regard the cleavage centrosomes as 

 new formations and as not related to the ' male ' centrosomes." 



Wheeler ('97) finds no sperm-aster in Myzostonia, and though 

 Kostanecki ('98) has since observed its appearance exception- 

 ally in this Qgg, he has not been able to trace it into the first 

 cleavage spindle. Wheeler finds that the cleavage centers are 

 furnished by the ^gg. 



In Pleiirophyllidia (MacFarland, '97) the sperm-asters dis- 

 appear entirely during the formation of the second polar body. 

 The centrosomes can no longer be distinguished. 



In AUolobopJiora foetida " the sperm attraction sphere is 

 present until the head of the spermatozoon begins to develop 

 into the male pronucleus, when it also totally disappears. Both 

 spheres, i.e., sperm and egg spheres, are absent during a rela- 

 tively long period {i.e., while the young pronuclei are develop- 

 ing) ; and when the pronuclei have attained their maximum 

 size and are in contact, two attraction spheres appear again in 

 the cytoplasm, and the cleavage spindle is formed." (Foot, '97, 

 p. 8ii). 



In Prostheceraeus (Klinckowstrom, '97) both egg and sperm 

 asters disappear completely before the union of the germ- 

 nuclei. 



In Cerebratithis (Coe, '99), "after the germ-nuclei are nearly 

 in contact, the sperm-asters have served the purpose for which 

 they were intended, and completely disappear in a manner 

 quite similar to that of the aster remaining in the egg after 

 the formation of the second polar body. Even the centro- 

 somes are lost from sight in most cases." Exactly the same 

 thing is true of Physa (Kostanecki and Wierzejsky, '96). 



In Echinoderms the sperm-asters become much less distinct 

 at the time of meeting of the germ-nuclei. Coe states ('99, 

 p. 455): "Observations which I have made on the eggs of 

 EcJiimis, of Sphaerechinus, and of Strongylocentrotiis, lead me to 



