MUSEUM OF COMPARATIVE ZOOLOGY. 369 



phological importance ! For this indistinguishable " physiological nu- 

 cleus " TschistiakofF employs the term pronucleus. 



In Isoetes this "pronucleus," under the influence of water, exhibits, 

 according to TschistiakofF (col. 20), the form of an ellipsoid, or, rather, 

 appears to be composed of two cones placed base to base ; the long axis 

 corresponds with that of the cell ; the substance of the pronucleus is 

 differentiated, so that one sees upon its surface more or less compact 

 lustrous streaks, which are arranged as meridians. These streaks 

 (" Differencialia " ) are apparently spindle fibres, and his " pronucleus 

 striatus " is the so-called spindle figure, or nuclear spindle. In a more 

 advanced stage, he continues, an equatorial welt of still more compact 

 substance is to be observed on the surface of the " pronucleus." This is 

 simply a plasmatic plate composed of projecting papillae, through which 

 the plasm is divided in its physiological centre. At this time there are 

 two small protoplasmic spheres near the poles of the " pronucleus " 

 [central areas of asters f\ which are soon converted into small vacuoles 

 [the equivalents of Biitschli's multiple nuclei?], — the "pronuclei" of 

 the two new parts. The plate which divides the " pronucleus" appears 

 in the form of a welt, because it is more compact than the rest of the 

 " pronucleus." This structure subsequently extends to the other parts 

 of the protoplasm ; the division proceeds from the centre toward the 

 periphery. The division of the protoplasm ensues in consequence of the 

 molecules becoming grouped according to their polarity. From this it 

 is evident that the groups, being in nature alike, must separate by rea- 

 son of the force of mutual repulsion. 



The formation of macrospores out of the mother cell in the case of 

 Isoetes is similar to the formation of spores of Anthoceros. The mother 

 cell has a morphological nucleus. The groups of starch granules are not 

 held, as formerly,* to be nuclei ; the secondary nuclei are not formed 

 in the presence of the primary nucleus, altliough the masses of starch 

 are. In the vicinity of the latter, innumerable transparent protoplasmic 

 filaments diverge in all directions. By the crossing of these filaments in 

 the middle of the cell there arise compact uniform plates [cell plates], 

 which, like the starch granules, have a tetrahedral arrangement, and 

 which serve, by their fission, to divide the cell into four portions. Four 

 secondary nuclei are found in the centre of the mother cell, each corre- 

 sponding to a cluster of starch granules. They are, however, much 

 nearer the planes of division, and are formed in the interior of the pri- 

 mary nucleus, which is subsequently dissolved. 



* See Nuovo Gicirnale Botanico Italiano, Tom. V. 

 VOL. VI. — NO. 12. 24 



