No. I.] THE EGG OF UMAX AGRESTIS. 227 



trace their origin to a middle-piece in the spermatozoon. The 

 time of their appearance is variable, but they usually are not 

 visible, even if present, until the Q.^g and sperm-nucleus have 

 reached their maximum size. There is no reason to believe 

 that asters that appear during the early fertilization stages in 

 the egg of Limax are temporarily lost only to appear later, 

 just before the formation of the segmentation spindle, as in 

 Allolobophora foetida. The asters once formed in connection 

 with the sperm-nucleus persist, but only in very exceptional 

 cases are they formed at all before the apposition of the Q.^g 

 and sperm-nucleus. Crampton has recently described a some- 

 what similar case in the egg of an Opisthobranch, Bulla, in 

 which the centrosomes do not appear in the fertilized egg until 

 the time of apposition of the pronuclei. 



VII. Summary. 



The centrosome in the q.%^ of Limax agrestis appears under 

 different forms during the maturation of the o.^^. Sometimes 

 the centrosome appears as a group of granules, as in the archi- 

 amphiaster stage. The granular centrosome at each pole of 

 the archiamphiaster is often divided into two distinct groups 

 or centrosomes, each of which is in turn composed of granules 

 that seem to be connected with each other, and appear dumb- 

 bell-shaped. Sometimes the centrosomes appear as single 

 granules, as in the centrosomes of the second maturation spin- 

 dle. The centrosome is seen as a large spherical body after 

 the extrusion of the second polar globule. The centrosome then 

 becomes granular and finally is resolved into a reticulum without 

 dividing, after which no definite centrosome can be detected. 



The character of the entire centrosphere changes during 

 the maturation stages. First, the centrosphere appears as a 

 series of concentric zones ; after this as a deeply staining cen- 

 ter with a single limiting zone ; then as a reticulated sphaere 

 with a large homogeneous body in the middle ; and finally as a 

 reticulated sphaere. The egg-centrosphere then disappears. 

 The various zones of the centrosphere are formed from the 

 cytoplasmic reticulum of the egg. 



