508 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
his observations. The centrosome arises exclusively from the spermato- 
zoon, as Boveri supposed. When the spermatozoon reaches the centre 
of the ovum, its protoplasm begins to break up and the centrosome is 
disclosed ; it separates from the rest of the spermatozoon, and comes to 
lie between the approximated pronuclei, where it divides into two. At 
the time when the pronuclei exhibit the same size and appearance, the 
connecting line between the two cenlrosomes is at right angles to the 
line connecting the pronuclei. 
The cytoplasm of the ovum has an exquisite honeycomb structure, 
and the so-called archoplasm of Boveri, or attraction sphere of van 
Beneden, is nothing but vacuolar protoplasm free from yolk, and aggre- 
gated around the male pronucleus or centrosome. There is a distinct 
alveolar layer round the periphery ; the polar and spindle rays are due 
to a particular disposition of the honeycomb structure in longitudinal 
lines. Even the spermatozoon and the centrosomes show the honey- 
comb structure. Nothing was seen of van Beneden’s polra’ or sub- 
equatorial circles. 
Mollusca. 
Archetype of Mollusca.*— Prof. A. E. Yerrill has a short notice on 
the Mollusc an archetype considered as a veliger-like form, with a dis- 
cussion on certain points in Molluscan morphology. He commences 
with calling attention to the well-known schematic mollusc of Bay 
Lankester. Prof. Yerrill is satisfied that that diagrammatic form by no 
means represents the primitive stage of the Mollusca. Even if Lan- 
kester’s type represented the primitive form of true Gastropods, it 
would seem impossible to derive from it a Cephalopod or a bivalve, for 
the parts are represented as so highly modified and specialised that it 
would require us to imagine a backward development in order to find a 
starting point from which the groups just mentioned might have been 
derived. A large number of Mollusca pass through a veliger stage, 
and it is in this that the author thinks we must look for the Molluscan 
archetype. The facts, which he summarises very briefly, lead him to 
the conclusion that ail Mollusca have been derived from free-swimming 
forms similar to modern yeligers and pro-veligcrs ; each of the great 
classes and perhaps some of the sub-classes became differentiated while 
still having veliger-like forms and modes of life; in other words, the 
primitive bivalves, Scaphopods, and Gastropods, when adult, were all 
small free-swimming forms furnished with a ciliated locomotive organ 
similar to the velum of veligers. Some of thepe swimmers may have 
developed directly a swimming foot adapted to a continued free-swim- 
ming existence, and thus the primitive Pteropods may have originated. 
In others, the primitive foot may have developed directly into a creeping 
disc or into a sucker-like disc for adhesion while resting. In the case of the 
Ileteropods, which are free-swimming Gastropods, with the foot in the 
form of a median fin, we often find a small cup-shaped sucker on the 
foot. This may be the survival of a condition that was common among 
primitive Gastropod veligers. Indeed, the Ileteropods, as a group, may 
be of very ancient origin, and not derived, as is often stated, from normal 
Gastropods in comparatively recent times. The primitive Cephalopoda 
* Amer. Journ. Sci., ii. (189G) pp. 91-9 (14 figs.)* 
