CELL-ORGANS OF SEA-URCHIN EGG 581 



A striking difference is presented by the form of the nucleus 

 after copulation of the nuclei and in artificial parthenogenesis. 

 In normal fertilization it is always oval: in the parthenogenetic 

 egg, nearly always spherical. I am not able to explain this dif- 

 ference or even to trace its causal relation to any other fact. 

 The nucleus of a normally fertilized egg is oval at the time, 

 when a single uniform radiation surrounds it (fig. 14) ; the nu- 

 cleus is oval, when the radiation is bipolar (fig. 16). Since the 

 poles of the nuclei are often occupied by nucleoli, it seemed nat- 

 ural to think of a possible relation between the form of the nu- 

 cleus and the position of nucleoli. If, however, the coefficient 

 of surface to mass may offer in the zones of greater convexity of 

 the nuclei more favorable conditions for resorption, the form of 

 the nucleus would be the primary factor. Large masses of ab- 

 sorbed substance appear indeed in the zones of greater convexity 

 as the sequence of the oval form of the nucleus, yet these accumu- 

 lations of chromatin may also be formed in other parts of the 

 nucleus, as figures 15 and 18 show. 



The oval form of the nuclei appears soon after copulation of 

 the pronuclei and the plane of their union has no relation to 

 either diameter of the oval segmentation nucleus. The nucleus 

 of figure 14 is oval; the plane of union being in this case parallel 

 to its longer diameter but the longer diameter of the nucleus is 

 often perpendicular to the fusion plane. It might be asked 

 whether the change in the form is possibly due to a specific influ- 

 ence of the centrosomes. Yet immediately after copulation 

 there could not be found at the poles of innumerable eggs studied 

 any structures resembling centrosomes. Neither is it possible 

 to trace a causal connection between radiations and nucleus 

 form, and if such exists the form of the nucleus must be consid- 

 ered again as the primary factor. The single radiation, which 

 develops around the segmentation nucleus (fig. 14), is invariably 

 followed by a rearrangement of the cytoplasm into a bipolar 

 radiation around the two poles of the oval nucleus (fig. 16). 



