The maturation and fertilization of the egg of Cerebratulus. 437 



cell even after the formation of the polar bodies. It is difficult to 

 determine in what manner the absorption of the nucleolus takes place, 

 and what eventually becomes of its substance. To all appearances, 

 however, it fades away as if by actual solution. 



The First Maturation Spindle. 



At the time the egg leaves the body the large germinal vesicle, 

 as above described, lies eccentrically in the egg, and is surrounded 

 by a conspicuous membrane, generally perfectly smooth and without 

 sign of indentation. On coming in contact with the sea-water, however, 

 whether the egg be fertilized immediately, or not, two slight in- 

 dentations of the nuclear membrane appear about 60" — 120° apart 

 (Fig. 3). These are usually found on that side of the nucleus which 

 lies nearest the periphery of the egg, but this is not invariably the 

 case, for occasionally one or both of them appear on the side nearest 

 the center. In each of these indentations, which are at first very 

 minute, is seen an exceedingly small, spherical body which stains in- 

 tensely black with iron haematoxylin and is, as its future shows, the 

 centrosome of one of the asters of the first maturation spindle. This 

 centrosome is the center of a few delicate radiations (Fig. 2) which 

 apparently consist merely of a slight thickening of the reticulum of 

 the cytoplasm along certain radiating lines. The centrosome is sur- 

 rounded by a very sHght, homogeneous sheath of less deeply stained 

 protoplasm, and it is from this sheath, apparently, that the radiations 

 diverge. The two asters are widely separated when they first make 

 their appearance , and both appear at about the same time. This 

 might seem to indicate that they have an independent origin as 

 Mead (20) has described in Chaetopterus. When the eggs of this 

 Annelid are freed from the surface of the ovary, the nodes of the 

 cytoplasmic reticulum "become less frequent and still more prominent, 

 until at length a large portion of the reticulum is transformed into a 

 multitude of small asters (secondary asters). Frequently one can 

 count from fifteen to twenty very distinct asters in a single section. 

 It is not long before two of the asters become predominate (primary 

 asters). Their rays increase in number and length, apparently at 

 the cost of the secondary asters , for the latter gradually evanesce 

 with the further development of the former, and at length the cyto- 

 plasm possesses only two well-marked centres of radiations. They 

 are destined to be the asters of the first maturation spindle. The 

 initial predominance and further growth of the primary asters certainly 



