MUSEUM OF COMPARATIVE ZOÖLOGY. 195 
Pagurus striatus, and P. callidus (Planche VII. Figs. 244, 245). A nu- 
clear plate is here formed, both in perfect mitosis and in degenerated 
mitosis; but in the former instance & spindle is formed, and the chromo- 
somes segment individually, while in the latter the plate divides in éto 
by constriction, without the help of a spindle, This modified type of 
mitosis, if we may so regard it, Carnoy considered as the result of 
degradation (pp. 316, 317), inasmuch as it appeared only in old sperm 
mother-cells after spermatozoa had become numerous in the testis. 
This accords with the earlier view that direct division is concomitant 
with senescence of the nuclei, based especially upon nuclear division in 
plants (Schmitz, ’79, Johow, ’81). I have regarded this as a possible 
explanation of the occurrence of amitotic division in the embryonal 
envelopes of the scorpion, for these tissues are temporary structures 
which obviously are near the end of their functional activity. This 
explanation, however, will not fit all cases ; for instance, the occurrence 
of amitosis in embryonic cells, and its prevalence in the testicular cells 
of some Isopods, already mentioned. 
The hypothesis advanced by Chun seems to throw light upon many 
of the cases of amitotic division which are referable to a sort of bud- 
ding or branching of the nucleus, carried to such a point that the 
buds or branches become constricted off as separate nuclear elements. 
These cases are, of course, not to be confounded with a disintegration 
of the nucleus, such as takes place in the macronucleus of Infusoria 
after conjugation, and sometimes in the degeneration of tissues. The 
distribution or extension of nuclear substance in the cytoplasm, whereby 
the surface of the nucleus is increased, is an event of frequent occur- 
rence. It is seen in the many forms of lobed nuclei, such as those of 
the ovarian capsules of Amphibia (see Flemming, ’82), and in those of 
leucocytes ; in hollow or perforated nuclei (giant cells); in branched 
nuclei (spinning glands and Malpighian vessels of Lepidoptera) ; and in 
the band-shaped and moniliform nuclei of many Infusoria. These pecu- 
liar shapes are evidently produced by the activity of the nucleus itself, 
probably correlated with a special function of the cytoplasm. From 
the deeply incised lobation or band-shape of such nuclei it is an easy 
stop to the formation of separate smaller nuclei by the deepening of a 
constriction already formed. Such daughter nuclei will as a rule be 
irregular in shape and unequal in size ; but if their production subserves 
a definite and important function, we should expect that in some cases 
their formation would become a regular process, governed by definite 
laws. It is possible that the more symmetrical kinds of direct division 
