MUSEUM OF COMPARATIVE ZOOLOGY. 293 



Owing to both these changes the contour of the nucleus is less distinct 

 than in the unfecundated egg. Its changes of form lead after a time to its 

 permanent elongation. Its two poles are occasionally truncate, so that, 

 in osmic acid preparations, it has the form of a cask. With this treat- 

 ment the nucleus is homogeneous. Meanwhile the poles of the nucleus 

 have become the centres of an accumulation of homogeneous substance, 

 which forms at first a small area, and then enlarges in all directions. 

 The formation and enlargement of the areas is accompanied (1.) by an 

 arrangement of the yolk granules in rays directed toward the nuclear 

 poles as centres, and (2.) by the growth of the suns thus formed by means 

 of a peripheral elongation of their rays. The poles of the nucleus be- 

 come in the living egg indistinct, and finally the nucleus suddenly disap- 

 pears ; but eggs treated with acids teach that the nucleus assumes the 

 form of a bent spindle, each of whose tips appears as a conspicuous dark 

 granule in the centre of its area. Later, the thicker middle portion 

 of the spindle exhibits a number of dark, coagulated, intensely stained 

 rods (Stabchen), lying parallel to the long axis of the spindle, and ap- 

 pearing in optical section of the latter as a circular cluster of granules, 

 which in Hert wig's figure (Fig. 27. a and d) appear evenly distributed 

 over the whole area of the circle. From the intense staining it is 

 concluded that these rods consist of condensed nuclear substance, on 

 account of which they are collectively named the middle zone of thick- 

 enings (mittlere Verdichtungszone). This structure is referable to a 

 process of difterentiation in the nucleus similar to that which takes place 

 in the formation of nucleoli (p. 414). 



After the apparent dissolution of the nucleus, the two suns are con- 

 nected by a narrow non-granular band, which occupies its place (dumb- 

 bell figure) ; the peripheral ends of the rays extend either to the surface 

 of the yolk, or to a plane (Theilungsebene) perpendicular to the middle of 

 the band, and the deep ends approach the centres of the suns with such 

 want of uniformity as to give the homogeneous area an irregular outline. 



The nature of all such " Radienfiguren " is explained (pp. 415, 416) as 

 resulting from a force which is exercised by the nucleus and expresses 

 itself in an attraction of the homogeneous protoplasm, so that the radial 

 arrangement of the yolk granules is only the optical expression of the 

 disposition of the protoplasm in which they are imbedded. The granules 

 are replaced by the protoplasm in the vicinity of the nucleus, since the 

 latter exercises no attractive influence upon them. The attractive force, 

 at first operating uniformly in all directions about the "Furchungs- 

 kern," is distributed at the time of the lengthening of the nucleus to its 



