MUSEUM OF COMPATIATIVE ZOOLOGY. 339 



the meniscus form, during which changes they move in opposite di- 

 rections, as if repelHng each other. The spindle fibres gradually 

 disappear. 



At the completion of cleavage a cluster of four to six refractive " nu- 

 cleoli " is formed in each of the areas, and the latter at that time begin 

 to approach, again passing through the same series of forms, but in the 

 reverse order. 



Although not' directly stated, it is to be inferred that these nucleoli 

 do not fuse until the formation of the second segmentation spindle. 



In the fourth instalment of his " Beitrage, " etc., 0. Hertwig ('78") 

 has devoted some attention to the phenomena accompanying the first 

 segmentation. 



Living eggs of Mitrocoma seem to become enuclear by the sudden 

 disappearance of the mutually flattened pronuclei, but the use of acetic 

 acid establishes the presence of a fibrous spindle which is eccentric in 

 position and is accompanied by radial arrangements of the yolk at its 

 tips. The division begins, about two hours after fertilization, with a 

 furrow which makes its appearance at a point of the surface directly over 

 the spindle, and is accompanied by the formation of secondary smaller 

 folds in the cortical substance at right angles to the segmentation fur- 

 row. A number of small vacuoles — the metamorphosed halves of the 

 nuclear spindle — make their appearance in each segment when the fur- 

 row has advanced as far as the middle of the yolk. The portions of the 

 yolk last separated are those lying diametrically opposite the side occu- 

 pied by the spindle (p. 183). 



The observations on Tiedemannia (p. 205) are more interesting since 

 the origin of the amphiaster is more successfully traced. The pronuclei 

 remain a long time close to the surface in an unaltered condition. At 

 length their nucleoli become disintegrated into clusters of smaller gran- 

 ules, which collect themselves on either side of the conjugating surfaces 

 of the pronuclei (Taf XL Fig. 5). Then two systems of faint rays make 

 their appearance at opposite edges of the surface of contact. Suddenly 

 the contours of the nuclei become indistinct, and both the vacuolar 

 spaces suddenly disappear, probably through a mingling of the surround- 

 ing protoplasm with the nuclear fluid. In the homogeneous protoplasm 

 one may still barely recognize the two systems of rays at some distance 

 from each other. The division follows much as in the medusa alluded 

 to above. 



Although the contour is figured somewhat less boldly where the two 

 nuclei are in apposition, it is not stated that the nuclei actually unite 



