244 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



pretation I have given : — that the centrosomes of the first cleavage 

 spindle are related to the sperm-nucleus alone. Although one centro- 

 some is often found very near the egg-nucleus, and even extends astral 

 rays toward it, I have never found both centrosomes cooperating to involve 

 the egg-nucleus in a spindle before the sperm-nucleus was involved. This 

 seems to me a point of vital importance, precluding, as it does, the possi- 

 bility of even one centrosome, being of other than spermatic origin ; for 

 if in Limnsea elodes one of the centrosomes comes from the egg, then 

 the incipient first cleavage spindles would be as likely to involve the 

 egg-nucleus as the sperm-nucleus first. 



SUMMARY. 



The centrosome and the centrosphere are extremely variable struc- 

 tures, both in size and in their reaction to stains. 



In the processes leading to the formation of the first polar cell in 

 Limax maximus, no centrosome is visible. The astral rays apparently 

 begin in the thickened wall of a pale centrosphere. 



After the formation of the first polar cell, and also after the formation 

 of the second polar cell, the centrosphere is found to be greatly enlarged. 

 This condition is characteristic of both Limax maximus and Limngea 

 elodes. 



In the egg cell of Limnsea elodes great variation in the condition of 

 the centrosome and the centrosphere is observed. Two facts tend to 

 show that the centrosphere is not a permanent organ of the cell : First, 

 the centrosphere is sometimes invisible on account of the increased area 

 at the centre of the aster which reacts to the stain, and, secondly, astral 

 rays beginning in the centrosome continue through the centrosphere. 

 The centrosphere then appears to be no more than a region of thicken- 

 ings in the astral fibres. 



The centrosome in the first maturation spindle of Limnsea, when it is 

 visible, varies from a very minute granule to a condition in which its 

 diameter is as great as the transverse dimension of the spindle itself, 

 or even greater. In the first maturation spindle of Limnfea the centro- 

 some was invisible in only a few cases, the astral rays usually giving 

 evidence of its existence. The centrosome of the second maturation 

 spindle of Limnsea never appeared as a large structure ; the number of 

 preparations, however, was limited. 



In both Limax and Limnsea after the formation of the second polar 



