i6 ATLAS OF THE FERTILIZATION AND KARYOKINESIS OF THE OVUM. 



III. APPROACH AND CONJUGATION OF THE NUCLEI. GROWTH AND FISSION OF THE ASTER. 



After tlie formation of the aster the two nuclei ajDproach anci finally unite with one another, as a rule not far from 

 the centre of the egg. The sperm-nucleus is usually preceded by the aster (Text-fig. XL, Plate IV., Phototype 14), 

 but may lie beside it. Its path is not directly towards the egg-nucleus, but towards a meeting-point which usually lies 

 near, but not at the centre of the egg. The egg-nucleus likewise moves towards the meeting-point, but traverses a 

 shorter path than the sperm-nucleus, since it does not begin to move until the sperm-nucleus has advanced some 

 distance. As a rule the aster meets the egg-nucleus first, and flattens down against it. The two nuclei then come 

 into contact and fuse together to form the cleavage- or segmentation-nucleus^ and this thereupon moves slowly to its 

 final position (often at some distance from the meeting-point) which is distinctly excentric and never precisely at the centre. 

 Meanwhile the aster divides into two halves (Text-fig. XII. A, B), which place themselves at opposite poles of the 

 cleavage-nucleus (see also Text-fig. I. D). 



The movement of the aster through the egg-substance presents some interesting mechanical problems. The 

 structural relations of the astral rays to the cyto-reticulum are such that a bodily movement of translation on the part 

 of the entire aster would seem to be impossible. What seems to take place is a translational movement of the central 

 mass and a concomitant progressive rearrangement of the surrounding reticulum, comparable with the rearrangement 

 of iron-filings in a moving magnetic field. 



Plate III. Phototype ii. 

 Inward Progress of the Sperm-nucleus. Delayed Rotation. 



The sperm-nucleus has advanced some distance into the egg, but still lies in a tangential position, the rotation 

 being only about 90°. One of the astral rays is sharply in focus throughout nearly its entire length and may be 

 traced nearly to the egg-periphery. The other rays, though beautifully distinct in the preparation, appear indistinct 

 since they lie out of the focus. 



The chromatic portion of the egg-nucleus is here shown with especial clearness, the chromatic membrane 

 (peripheral chromatin) with its pores being very distinct, and also the internal jDortion of the network. The clear spaces 

 are filled with the nuclear sap traversed by transparent strands of linin clearly visible in the preparation but not 

 shown in the photograph, 



Plate III. Phototype 12. 

 Double Fertilization or Dispermy. 



This specimen represents a case in which, as often happens, two spermatozoa enter the egg at the same 

 instant. One of the entrance-cones is clearly shown. Each sperm-head has developed an aster, and both are 

 advancing towards the eg2;-nucleus. 



Observations on the living egg show that both sperm-nuclei unite with the egg-nucleus. Each aster afterwards 



