310 BULLETIN OF THE 



Owing to the opacity of the cells in Unio (pp. 213-216) they were 

 studied with the aid of sections. Only embryos consisting of about 

 twenty cells were used. The nuclear plate is found by study of cross 

 sections to be formed, not of an annular series of granules (Korner- 

 kranz), but by a continuous disk (durchgehende Kornerscheibe). The 

 author does not mention any inequality in the distribution of the 

 granules, such as is exhibited in Limax, nor does his figure [loc. cit, 

 Taf. VIL Fig. 18 6) make the granules of the "Kernplatte" more con- 

 spicuous than those of the surrounding protoplasm. The poles of the 

 spindle are much more marked, on account of their great refractive 

 power, than in plant cells. The rays of the protoplasm converging 

 toward them are very distinct in animal cells, hardly traceable in plant 

 cells. The two segments resulting from a division of the nuclear plate 

 are found in different cells at varying distances from each other. In- 

 terzonal filaments are neither increased in number, nor suffer a lateral 

 expansion, as in plant cells. In some of Strasburger's figures (e. g. 

 Taf. VII. Fig. 20) both ends of the spindle appear broadly truncate after 

 the separation of the segments of the nuclear plate, and each trun- 

 cate face occupying the centre of its aster is marked by a conspicuous 

 structure in which the nuclear fibres terminate. 



Following this condition is a stage in which the new nuclei are homo- 

 geneous. The latter never occupy the centre of the sun, but often ro- 

 tate, as it were, about the former pole without reaching it. In this w^ay 

 two contemporaneously formed nuclei may eventually lie (as in Stras- 

 burger's Fig. 10, Taf. VIII.) farther apart than the centres of the two 

 asters. In all animal cells which he has had the opportunity of study- 

 ing, Strasburger finds that the new nucleus becomes at first homogene- 

 ous by the confluence of all the componcnts'of each half of the maternal 

 nucleus (p. 226), that its definite formation beginning on the side toward 

 the equatorial plane advances to the poles, and that the portion defi- 

 nitely perfected (equatorial portion) is distinguishable from the more 

 homogeneous by its low power of refraction and by its nucleoli ; further, 

 that this equatorial portion has generally been taken for the whole nu- 

 cleus. The nucleus may arise by the union of two closely situated vacu- 

 oles, but a greater number than two had never been seen. At the time its 

 definite formation (Ausbildung) is completed, the nuclei are more or less 

 pear-shaped, with the pointed ends occupying the centres of the asters. 

 The rays disappear ; the nucleus becomes rounded and a membrane is 

 formed ; it still continues to enlarge, probably taking its food from the 

 surrounding homogeneous protoplasm (pp. 219-221). 



