MUSEUM OF COMPARATIVE ZOOLOGY. 333 



Hoffmann ('77) was unable to follow any of the internal changes ac- 

 companying the segmentation of eggs of Malacobdella (p. 23), but found 

 that a constant change of the external form of the yolk accompanies tho 

 o;u'lier segmentations. In Clepsine the same author ('77") has by means 

 of sections been able to see something of the stellate figures (of which 

 his Fig. 10 gives a rather diagrammatic view), but nothing of a spindle 

 figure during segmentation (p. 35). 



Selenka ('78) has given a preliminary account of observations on liv- 

 ing eggs of Toxopneustes variegatus. 



During the union of the sperm- with the egg-nucleus to form the pri- 

 mary cleavage nucleus, the plasma, which constituted the central area of 

 the radial figure surrounding the former, in part flows around the egg- 

 nucleus, and thus gives rise to a second central area. These two central 

 areas diverge, the cleavage nucleus becomes ellipsoidal and suffers a 

 metamorphosis, while the yolk becomes flattened in the direction of the 

 axis of the nucleus. The latter exhibits at each pole one, and then sev- 

 eral deep incisions, resulting in a longitudinal fission of its whole sub- 

 stance into some twelve cylinders (Kerncylinder). An equatorial nuclear 

 plate arises and splits into two plates, each composed of twelve " Kern- 

 stabchen," or " Vorkerne," arranged in a circle. These migrate to the 

 respective ends of the nucleus, which has meantime become an elongated 

 cylinder, and here they melt together, first into six, then into two, and 

 finally, but slowly, into a single new nucleus, which rapidly grows to 

 double the size it had at first. A constant increase in the mass of the 

 " Vorkerne " accompanies their union, and is caused by the absorption of 

 the contents of the "Kerncylinder," which is really composed of pro- 

 cesses of the " Vorkerne.'' The flattening of the yolk is followed by a 

 lengthening in the same axis, and finally it assumes the constricted form. 

 The first constriction may, however, disappear before it has accomplished 

 the separation of the halves, to reappear only when two more nuclei 

 have been formed and a second constriction has made its appearance, in 

 a plane perpendicular to that of the first. Selenka considers this, as 

 well as the usual method, normal, and is inclined to seek an explanation 

 of the former in the hastened division of the primary cleavage nu- 

 cleus. 



AuERBACH ('77) has given, in a paper which I have not been able to 

 secure,* an explanation of the striation of the spindle. It does not neces- 

 sarily originate in the same manner in all cases. If the granules imbed- 



* This account is taken from Hofmaun u. Schwalbe's Jahresbericht, Bd. VI., 

 Anat. Abth., p. 25. 



