ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
095 
and in tlie character of the nuclei. It is found that in several species 
the microspheric form has many comparatively small nuclei, while the 
megalosplieric form has a single large nucleus. The latter form is much 
more numerous than the former. 
The megalosplieric form has, in at least seven genera, been seen to 
arise as a young individual already invested by a shell, produced in the 
terminal or peripheral chambers of the parent. The parent may be 
megalo- or microspheric. Foraminifera, in certain conditions, give rise 
to active swarm-cells. 
The important question arises, are these two forms distinct from their 
origin, or is one a modification of the other. Mr. Lister adduces reasons 
in favour of the first explanation, and suggests that the two forms are 
members of a recurring cycle of generations. 
From what has been seen in Orbitolites complanata , in which both 
micro- and megalospheric forms have been found with the young of the 
megalospheric form in their brood-chamber, it is necessary to suppose 
that, in some genera at any rate, the megalospheric form may be repeated 
for one or more generations, before the microspheric recurs. No 
evidence of such a repetition has been furnished by Polystomella. 
The view that the life-history of the Foraminifera comprises more 
than one generation is in harmony with the fact that the nuclear history 
of the two forms in Polystomella appears to present resemblances to what 
have been described by Brandt in TJialassicolla ; here the individuals 
fall into two sets, one of which produces isospores, and the other aniso- 
spores ; and these are regarded as an asexual generation alternating with 
a sexual. 
In the megalospheric form of Polystomella a simultaneous division 
of the nuclei by karyokinesis was observed immediately before the 
formation of the reproductive elements. This is a phenomenon of very 
general occurrence, and it is probable that it is akin to the division of 
the micronucleus which precedes conjugation in the Infusoria, and to 
the division of nuclei which occurs in the maturation of the reproductive 
elements in the higher forms of Animals and Plants. 
Saccammina.* — Dr.* L. Bhumbler gives a most exhaustive account 
of Saccammina spliserica M. Sars, which he obtained in abundance from 
the Baltic. The shell-wall is solid, with only one or two very minute 
apertures at the end of the papilla-like projections. Its cementing sub- 
stance is horny, not chitinoid, and the brown colour is due to impregna- 
tion with an iron salt. The youngest tests differed considerably from 
the older forms ; what has been called Psammosphsera fusca F. E. Schulze 
is a second stage which is formed after the destruction of the primitive 
test ; from this, by the formation of a pylome-tube, &c., the final form 
arises. Besides strayed Metazoa, six different unicellular organisms 
were found within the tests, one the true tenant, the others intruders. 
As to the structure of Saccammina , there is a definite peripheral layer, 
interrupted by the pseudopodia; special processes traversing the test 
have to do with building up the wall ; the true pseudopodia have a 
webbed structure and small granules ; the general substance of the cell 
is a spongy web, containing in its interstices particles taken in from 
* Zeitschr. f. wiss. Zool., lvii. (1894) pp. 433-536 (4 pis.), 587-617 (1 pi.). 
3 b 2 
