72 FE" COMPARATIVE EMBRYOLOGY OF THE RUBIACEAE 
Byxbee, * Williams, + and others, and it has been suggested by 
Strasburger that, in those spindles of which the ends do not reach 
the ectoplasm, or in the absence of other mechanical support, the 
pericaryoplasm (of Lawson) may serve that purpose. 
I have observed this zone in the plants under consideration, 
but in instances the poles of the mitotic figure reach the ectoplasm 
)۸ 11, fig. 2). In this event the zone thins out as it approaches 
the spindle pole. In the pollen mother-cell of Crucianella macro- 
stachya this zone has not been observed and the spindle appears to 
lie quite freely and without any special mechanical relations. 
The amount of variation in the observations of the topographic 
relations of the spindle ends leads to the suspicion that we are yet 
unable to form a satisfactory explanation of them. The most far 
reaching interpretation among the botanists is that of the Stras- 
burger school, and is based upon the mechanical conception that 
the spindle is made up of contractile fibers (Zug-fasern) which, in 
order to pull apart the chromosomes, must have a more or less 
rigid mechanical support. This has been found in the ectoplasm, 
in the pericaryoplasm, in the vacuoles,f or the spindle ends may 
be buttressed by extra-nuclear supporting fibers. 
That such mechanical relations as described by Strasburger 
and others really exist we may not doubt. All the appearances 
lead irresistibly to that conclusion. But I am by no means con- 
vinced that we may infer from these appearances anything as to 
the intimate workings of the mechanism, and the contractile fiber 
explanation may have to undergo perhaps a considerable degree 
. of modification before it may be accepted as satisfactory. 
Reference has already been made in another part of this paper 
to the absence of cell walls between the megaspores. It will be 
seen from pl. Ir, fig. 20, and pl. 9, fig. 23, that the connecting 
fibers are formed, and the thickenings indicative of the cell plate 
are present. In Crucianella the presence of these fibers determines 
the appearance of the cytoplasm for some time (pl. 10, fig. 9). 
At the end of the third division in the same plant the nuclei are 
*'The Development of the eg si in the Pollen Mother- cells of 
Lavatera. Proc. Cal. Acad. Botany, III. 
The Origin of the LOUPE inb in e coerulea Linn. Proc. Cal. 
Acad. Botany, III. 1 : 1899. 
+ Strasburger, 1. c. 
