GYMNOSPERMS. 167 



Although the male nucleus is almost enclosed by the female, actual 

 fusion, according to Blackman, does not take place immediately, since 

 the membrane of the male nucleus is intact (Fig. 69, B). The mem- 

 brane soon disappears, but the chromatin of the two nuclei does not fuse 

 at this stage and no resting fusion nucleus is formed. With further 

 development the chromatin of each nucleus will give rise to a group of 

 chromosomes, which become arranged upon the spindle of the first 

 division after fecundation where they are seen to be split longitudinally 

 (Fig. 69, C). As has been pointed out for Ginkgo (Fig. 67, C, D) the 

 spindle seems to arise entirely within the limits of the female nucleus. 

 In Pinus laricio, according to Chamberlain, after the male nucleus 

 is within the nucleus of the egg, the chromatin of the two pronuclei 

 appear as two distinct masses in the spirem stage. Murrill finds that 

 in Tsuga canadensis both nuclei are in the resting condition when 

 actual fusion begins, but he seems to be of the opinion that the identity 

 of the male and female chromatin can be traced until the division of 

 the fusion nucleus, as will be seen from the following : 



The chromatin of each nucleus collects in the form of a thick knotted thread 

 near the center of the separating partition, and the two masses remain distinct 

 until the spirem bands begin to segment. Just before the spirems are formed 

 the separating membranes disappear and the nuclear cavities become united. 

 The spindle then arises in a multipolar fashion between and among the two 

 masses, twelve chromosomes being supplied from the chromatin of the sperm 

 and twelve from that of the egg, as described by Blackman for Pinus sylvestris. 



Ferguson finds in Pinus strobus that the two sexual nuclei do not 

 fuse in the resting stage. The male nucleus imbeds itself in the egg- 

 nucleus but does not penetrate its membrane. In each nucleus is devel- 

 oped a chromatin spirem and an achromatic reticulum. The nuclear 

 membranes now disappear, but the two chromatin groups remain 

 distinct until the nuclear-plate stage (Fig. 68, D). 



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 maternal nucleus. The spindle fibers appear to arise by a re- 

 arrangement of the achromatic nuclear reticula, and are evidently not the 

 expression of a special kinoplasmic substance. 



In the stage of the mature spindle of the first division following 

 fecundation in Pinus austriaca, the species examined by myself, no 

 distinction whatever could be recognized between male and female 

 chromatin. 





