306 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



polar figure. When these abnormal cleavage figures arise, segmentation 



is rarely completed, the egg remaining in a partly formed two-cell stage. 



In some capsules more than half the eggs were thus abnormal. In 



eggs in which the abnormal figures occur, I have found that the number 



of asters which unite to form these multipolar figures does not indicate 



a doubling of each extra male pronucleus, but that a single aster arises 



for each. In Hamiuea, then, a tripolar figure indicates that there were 



two male pronuclei ; a tetrapolar figure, that there were three male 



pronuclei in the egg. This ftict furnishes additional evidence in favor 



of the view that one of the cleavage centrosomes is associated with each 



pronucleus. 



(c) Discussion of Results and Literature. 



The only account of the development of Harainea that has appeared 

 is the preliminary report by Crampton ('97, p. 63), Avhich I quote in full : 

 " The observations were made upon the eggs of a species of Doris, 

 collected last summer on tlie Pacific Coast by Mr. Calkins, and upon a 

 species of Bulla [Hamiuea] which deposite<l eggs at Woods Hole during 

 the months of August and September. The results may best be sum- 

 marized by stating that a complete confirmation was obtained of the 

 account of fertilization given by Wilson and Mathews, Boveri, Hill for 

 sea urchins, Meade on Chsetopterus, Kostanecki and Wierzejski upon 

 Physa, etc. The sperm nucleus is preceded by the divided centrosome, 

 an aster, however, not being found till the union of the germ-nuclei. 

 The first polar spindle has at each pole a double centrosome, the second 

 maturation spindle but one. These are of great size, however, and the 

 one remaining in the egg finally disintegrates, the centrosome of the first 

 cleavage spindle being derived from the sperm. The germ-nuclei do not 

 fuse, but lie very close to one another, in contact." 



It is generally conceded now that it is unsafe to apply the term " centro- 

 some " to a body that shows no evidence of association with astral rays. 

 The fact that no rays were found by Crampton in connection with the 

 sperm centrosome renders his conclusions open to serious question. If 

 a centrosome does not precede the sperm head in its migration in the 

 egg, — and I believe that it does not, — then my observations in regard 

 to these changes agree with those made by Crampton. 



We may now inquire into the significance of the bodies in Haminea 

 (Bulla) which I have described as accessory asters.^ Similar conditions 



1 For a full description of the general subject of accessory asters, see Wilson 

 (:01^ pp. 560 ff., 580 ff ). 



