LIFE HISTORY OF PINUS I3I 



their cells. But if, as has been suggested, the egg-nucleus 

 functions as a manufacturer of nutritive material, may we not 

 find in this activity a feasible explanation of its greater size ? 

 The conjugating nuclei, always dissimilar in size, may or may 

 not be dissimilar in structure. 



The egg-nucleus becomes slightly convave on the side nearest 

 to the approaching sperm-nucleus. This nucleus imbeds itself 

 in the side of the egg-nucleus but does not penetrate its mem- 

 brane. A chromatic spireme arises, and a prominent achroma- 

 tic reticulum becomes apparent in each nucleus. Soon after- 

 wards the nuclear membranes entirely disappear. The two 

 chromatic groups remain distinct until the nuclear plate stage. 



Fertilization consist in Pimis in the union of two entire cells. 

 Cytoplasm fuses with C3^toplasm, but there is never any fusion, 

 as ordinarily understood, of the sexual nuclei. 



The spindle of the first division following fecundation always 

 lies between the conjugating nuclei and parallel with the outer, 

 free surface of the sperm-nucleus. It is multipolar in origin 

 and is probably derived equally from the paternal and the ma- 

 ternal nucleus. The spindle-fibers appear to arise by a rear- 

 rangement of the achromatic nuclear reticula and are evidently 

 not the expression of a special kinoplasmic substance. After 

 the formation of the daughter-nuclei, the greater portion, if not 

 all, of these threads pass into the cytoplasmic network. Dur- 

 ing metakinesis and later stages this spindle may vary from a 

 broad, multipolar diarch to a slender, bipolar spindle. The 

 chromosomes pass to the poles in the form of narrow U's. 



No individualized centrosomes or centrospheres have been 

 found to occur in connection with the first division following fer- 

 tilization. But the entire activity connected with this mitosis indi- 

 cates that the sperm-nucleus acting in the presence of the egg-cy- 

 toplasm is the agent which initiates and controls the division. 



The two segmentation-nuclei present a reticulated structure 

 in which the paternal and the maternal chromatin appear to be 

 completely fused. They divide in the upper part of the ^^^ 

 passing through practically the same steps as those noted for 

 the first division. The two chromatic spiremes formed in each 

 nucleus take up a position along the adjacent sides of the 



