ANATOMY AND PHYSIOLOGY. 
Moll . 41 
equator, fuse together and form triangular or hook-shaped structures 
attached by the apex to the equator. After division this structure 
retreats towards the centre, and the limbs of the hook form a closed 
figure, the accessory nucleus ( il Nebenkern ”). An analogous process is 
observed where the protoplasm does not divide. The framework of the 
coil, the spindle-fibres and the accessory nucleus have a genetic connection, 
being all modifications of the same element. The process may be summed 
up as follows : “ After the chromatin of the nucleus has divided into 
microsomata, these arrange themselves into regular curved rows ; the 
accessory body enters the nucleus and forms the framework of the coil, 
the latter as the spindle-fibres ; while the chromatin concentrates in the 
granular equatorial plates, the spindle-fibres come into direct connection 
with the protoplasm at the poles ; after division of the chromatin sub- 
stance, the resulting pole-plates form anew regular nuclei, and the 
‘ anaphasis ’ either repeats inversely the stages of the ‘ prophasis,’ or the 
accessory nuclear body arises directly from the spindle-fibres,” according 
to the extent to which the protoplasm takes part in the division. Carnoy 
and Gilson’s investigations are criticised. [2] Oogenesis and spermato- 
genesis in Avion. — In the earliest stages a primitive sexual-cell is a homo- 
geneous substance ; granules form in it and granular protoplasm arouud 
it. These form (a) primitive ova, (6) spermatogonia and basal cells, (c) 
nutritive yolk-cells, ( d ) reserve germs. The peripheral cells are ova, and 
the central zone spermatogonia, characterized by the presence of a single 
large nucleolus. The accessory body is an outgrowth of the nucleus ; it • 
has a distinct membrane, and the chromatin is irregularly distributed 
within it. The ova have a larger quantity of protoplasm and a larger 
nucleus ; they increase in number as in Helix . The nucleus contains the 
germinal spot and another round body, the nucleolus. The accessory 
nucleus disappears in the ripe ova, and the spindle-fibres are formed from 
unstained substance contained in the germinal spot. The mature ova lie 
in scattered groups with spermatogonia between them, the intermediate 
substance being composed of connective tissue cells ; the reserve germs, 
however, do not originate from these, but from the sexual cells. 
[3] Oogenesis in Helix. — The ova are formed at intervals, and when ripe 
exhibit a single germinal spot. The nutritive cells are at first in contact 
with the ova, but afterwards lie in cavities and cannot be distinguished 
from the surrounding protoplasm. There is no vitelline membrane ; 
Platner (1). 
In Arion empiricorum the period between fertilization and egg-laying 
is variable ; two yellow folds in the uterus mark the position where the 
albumen is deposited ; the cavity of the uterus contains many sperma- 
tozoa ; polar bodies are present, usually three, the result of the division 
of two ; they possess distinct membranes ; their formation regarded as 
a necessary prelude to fertilization, but independent of it. Sperm 
usually penetrates at the vegetative pole away from the germinal 
vesicle ; only one sperm seems to be effective, subsequent ones disappear ; 
the head followed by the tail approaches the ’ germinal vesicle within 
which nucleolus-like “ karyosomata ” have formed. These are united by 
unstained bridges which afterwards disappear. The head of the sperm 
1886. [VOL. XXIII.] D 4 
