26 
PROGRESS OP MICROSCOPICAL SCIENCE. 
by sometliing opaque ; and in every case of dog's spermatozoa and in 
a single case of man's, where no central thread was seen, it could be 
rendered visible. On the spermatozoa of the cat and hare there is a 
bound-up appearance at the situation of the middle piece. 
These general observations, together with the conduct of the sper- 
matic elements of the flying mouse, show that the central thread must 
be a peculiar element of the spermatozoa of mammalia ; and that the 
middle piece consists of a central thread and of a protoplasma cloak 
covering the same, which cloak is very often divided into little cubes. 
This cloak is a remnant of the original formative cell of the sper- 
matozoa. This appearance is common also on normal spermatozoa, 
and is often crowded in rumples on different parts of the middle piece, 
especially on the anterior end. 
The tail also of mammalian spermatozoa consists of a fine thread 
and protoplasma covering which is wanting in the extreme end of the 
tail. We can follow the centre thread in some flying mice quite to 
the end of the tail. 
In the night-flying mouse, cat, and hare, the extreme end of the 
tail is thicker than the central thread, and is always, in all mammals, 
thicker than the very thin throat. 
Secondly, the author speaks at length of the motion of these bodies, 
their kind and cause. 
In the triton and salamander the spermatozoa have a finny edge 
running along the whole length on both sides. These edges move in 
screw-like windings commencing at the back of the head and extend- 
ing to the tail in quick succession, and a steady and uniform motion is 
imparted to the body. 
In spermatozoa which have no finny edge the forward motion 
is produced by rapid turning upon the long axis. The extreme end of 
the tail goes round in circles, and in this way turns the whole body. 
The rate of propulsion is in proportion to the rapidity of tail-rotation. 
The tail does not always turn in the same direction. "When turning 
very slow the rotations can be counted. 
The author finally says : — The motion of the spermatozoa of mam- 
mals and other vertebrates is on the principle of a screw. The 
streams in the protoplasma rush to the end and cause the end to 
rotate, whence the whole body is turned and the forward motion pro- 
duced. — Virch. ' FJiysik. Med. Ges.' in Wurtzburg, vi., 3, p. 93. 
Observations on some Marine Rhizopods. — At the meeting of the 
Academy of Natural Sciences of Philadelphia, on March 16, Professor 
Leidy remarked that he had spent a short time last August at Noank, 
on the coast of Connecticut, where Professor Baird was then engaged 
in pursuing his inquiries and investigations as United States Com- 
missioner of Fisheries. Through the kindness of Professor Baird 
ho had been enabled to make a few observations on some marine 
Rhizopods. 
Some years ago, on the beach at Newport, R.T., he had noticed that 
the ripple marks of the sand were crested with white particles, which 
could be scraped up by the handful, and which he at first viewed as 
the pulverized debris of various calcareous shells. On closer exami- 
