MUSEUM OF COMPARATIVE ZOOLOGY. 153 



Pagurtis striatus, and P. callidus (Pknche VII. Figs. 244, 245). A nu- 

 clear plate is here formed, both iu perfect mitosis and in degenerated 

 mitosis ; but in the former instance a spindle is formed, and the chromo- 

 somes segment individually, while in the latter the plate divides in P.to 

 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 hrancJiing of the nucleus, carried to such a point that the 

 buds or branches become constricted oflP as sepai'ate 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 ovai-ian capsules of Amphibia (see Flemming, '82), and in those of 

 leiicocytes ; in hollow or perforated nuclei (giant cells) ; in branched 

 nuclei (spinning glands and ^lalpighian 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 

 step 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 pi-oduction 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 



