Rickett.—Fertilization of Sphaerocarpos . 255 
the Marchantiales, which latter group includes all the other Liverworts in 
which fertilization has been thus far described except Riella. Should this 
suggestion be verified, it is important from a phylogenetic standpoint to 
note that the history of fertilization in Sphaerocarpos and Riella resembles 
that found in the Marchantiales. 
If, as seems likely, there is no nuclear fusion, Sphaerocarpos constitutes 
in this respect a unique case among plants. Such a behaviour of the nuclei 
as seems to hold in Sphaerocarpos is characteristic of the gamete nuclei of 
many animals, reaching its extreme expression in such cases as that of 
Cyclops , wherein the male and female nuclei not only furnish independent 
groups of chromosomes, but these persist as two distinct groups, and can 
be followed through the division of the zygote, and in some cases through 
several succeeding divisions. Among plants, the closest approach yet noted 
to such a condition is found in Pinus Strobus, as described by Miss 
Ferguson ( 1904 ). Here the male nucleus is received in a concave depression 
in the surface of the (much larger) female nucleus. The chromatin is then 
organized into a spireme in each nucleus, while the boundary between the 
two is still very plain. The nuclear membranes then £ fade out leaving the 
two spiremes on the spindle. The distinction between the two parental 
groups of chromosomes becomes lost in the equatorial plate of the first 
division. 
Fusion of nuclei while the latter are in the spireme state has been 
described by several writers—as by Miss Weniger ( 1918 ) for Lilium phila - 
delphicum and L. longiflorum , by Sax ( 1916 ) for Fritillaria pudica (in rare 
cases). The separate formation of chromosomes in two groups by the male 
and female nuclei after they have completely fused has been reported by 
Blackman ( 1898 ) for Pimts silvestris, by Murrill ( 1900 ) for Tsuga canadensis, 
by Hutchinson ( 1915 ) for Abies balsamea , and by Sax ( 1918 ) for Triticum . 
Chamberlain ( 1899 ) figures an egg of Pinus Laricio which contains two 
separate spiremes. In many of the higher plants that have been studied, 
however (Chamberlain, 1916 ; Sax, 1916 , 1918 ), the two sexual nuclei fuse 
completely while in the resting state. This is apparently characteristic for 
the Thallophyta (a review of cases of fertilization in a number of the lower 
plants is given by Mottier, 1904 ). In Polysiphonia violacea , however, 
Yamanouchi ( 1906 ) reports that the male nucleus contains distinct chromo¬ 
somes when it fuses with the female nucleus ; the latter is in a resting con¬ 
dition. In the ferns, so far as known (Yamanouchi, 1908 ), the two nuclei fuse 
completely, and the fusion nucleus passes into a resting condition before 
the first division. 
Sphaerocarpos furnishes yet another instance in support of the doctrine 
of the individuality of the chromosomes, since the eight chromosomes that 
are formed separately in each nucleus are undoubtedly the same as those 
that pass on to the spindle and finally form the daughter nuclei of the first 
S 
