426 
NATURE 
[JUNE 27, 1912 
connection with those other cultures which failed of 
success. I am indebted to one of my students, Mr. 
Ronald Grant, for much help in the needful mani- 
pulations. 
Union of the two coelomic vesicles in front of the 
mouth took place about the twentieth day, while com- 
plete separation of the hydroccele from the left pos- 
terior coelom was accomplished by the thirtieth day, all 
the radial pouches being unmistakable before the end 
of the fifth week. Fixation was observed on the 
fifty-second day, but it appeared afterwards that one 
or two specimens must have attached themselves at 
least as early as the middle of the seventh week. 
Now, in the ninth week, my largest specimen measures 
“75 mm. across the disc, is provided with three or 
four pairs of sucker feet in each ray, has well- 
developed eye-spots, and can travel at the rate of an 
inch in five to seven minutes. 
Various abnormal larvze were observed, the most 
remarkable being three specimens with double hydro- 
coele. These were perfectly symmetrical externally, 
and also internally, except that the left hydroccele 
alone was provided with a hydropore. One of them 
was unfortunately lost, the second was preserved early, 
while the third reached a length of more than 2 mm. 
and entered on the stage of attachment. It then pre- 
sented a remarkable appearance—the two sets of 
hydroceele buds appearing as outgrowths on _ the 
surface; the arm-lobes arching round the posterior 
end of the body in the sagittal plane; the hydropore 
in the mid-dorsal line; the long processes of the 
ciliated band in great part absorbed; the mouth and 
cesophagus still open and in functional activity; the 
internal cavities apparently quite similar on both sides ; 
and the whole as symmetrical as the conventional 
dipleurula, to which indeed the mode of attachment 
by the preoral lobe and the slanting carriage of the 
body gave additional resemblance. 
As I watched the specimen after it became attached, 
the brachia, and partly also the sucker, were being 
used with great activity, and in such a manner that, 
during the thirty minutes I had it under observation, it 
travelled four millimetres across the bottom of its dish. 
When next I had the chance of looking at it, the 
specimen was detached and somewhat contracted, and 
fearing that it had suffered injury in the previous 
manipulation, I preserved it for future work. 
It is remarkable that the twenty-five brachiolariz 
available for examination provided me with three 
examples of double hydroccele. The culture had been 
made early in the season, at a time when the ovaries 
were distinctly unripe. It is open to suggest that 
these facts are directly related to one another, 
abnormal potencies that are ancestral in their deriva- 
tion being likely to be strongest in ova hurriedly 
matured. Under natural conditions, double hydroccele 
is apparently so rare in feeding brachiolariz that it 
has hitherto escaped record, although, as is well 
known, MacBride has directed attention to note- 
worthy instances of its occurrence in Asterina, Ophio- 
thrix, and Echinus (Q.J.M.S., vols. xxxviii., p. 368; 
li., p. 570; lvii., p. 235). J. F. Gemniiv. 
Embryological Laboratory, Glasgow University, 
June 10. 
Clouds and Shadows. 
Mr. Cyrm Crossianp’s description (p. 322) of 
great shadow bands cast across the sky at sunset 
interested me, for I well remember being impressed 
by a similar phenomenon when crossing a New 
Mexican prairie, with the sun setting behind the 
Rockies. I have seen the bands in England, but 
imperfectly. To a non-expert, like myself, Mr. Cross- 
land’s remark, ‘‘The shadows being cast by the 
No. 2226, vot. 89] 
reflected light of the glowing clouds in the west, not 
by the sun itself, of course,”’ presents difficulties. A 
mass of glowing cloud seems too extensive a lumin- 
ous source to cast definite shadows of peaks com- 
paratively near it. Further, the sun being beyond 
the cloud, the bulk of the sunlight reflected by the 
cloud would fall the wrong way. Long after the 
sun’s rays are cut off from the spectator, they will 
still be shining upon clouds high overhead, and 
therefore able to cast shadows, 
If we suppose the shadow rays described to be cast 
by the sun itself, then it is easy to explain the ap- 
pearance of the rays converging to the east, which 
puzzled Mr. Crossland. If the height, above the 
earth, of the under-surface of the cloudy stratus be 
roughly uniform, then this surface may be practically 
regarded as plane so far as it is visible to the spec- 
tator. At any rate, the curvature will be small, for 
the visible portion of the cloud canopy is a very 
small fraction of the sphere, concentric. with the 
earth, of which it forms part. The sun being prac- 
tically at infinite distance, the rays of shadow cast 
by it upon this overhead plane will be parallel, and 
hence, by the laws of perspective, will appear to con- 
verge as they recede from the zenith, or region 
nearest the spectator, to more distant regions east 
and west. : 
Perhaps it is not always realised how far clouds 
“on the horizon”? may be beyond the (terrestrial) 
horizon. It is quite an interesting little exercise to 
work out. Assume the earth to be a smooth sphere, 
and the lower cloud surface a smooth concentric 
sphere. Let a line be drawn from A, the eye, to 
touch the earth at B, and produced to cut the cloud 
sphere at C. We have, roughly, 
BC? =(4000+h)?—4000° miles, 
h being the height of the cloud above the earth. BC 
is independent of the spectator’s altitude. For a 
cloud-height of five miles (if I have worked it right) 
BC is about-200 miles; and for a cloud-height of 
half a mile, BC is 893 miles. 
The first volume of a great German work on 
meteorology was devoted to explaining why the popu- 
lar impression of the form of the sky is that of a 
flattened vault. If this is the general impression, it 
has struck me that it may be based on observation 
of the local cloud canopy rather than of the clear 
sky. ALIcE EVERETT. 
Milbourne Lane, Esher. ; 
POLITICS AND SCIENCE. 
E desire to call attention to two lectures de- 
livered by Professor Karl Pearson on 
March 12 and 19, and now published in pamphlet 
form (Dulau & Co., Ltd., 1s. each). The first 
is entitled “Tuberculosis, Heredity and Environ- 
ment,” the second, “Social Problems: Their 
Treatment, Past, Present and Future.” 
The first lecture contains an account of the 
recent work carried out at the Galton Laboratory 
for National Eugenics on the subject of tuber- 
culosis, and it is deeply to be deplored that the 
evidence therein contained was not made public 
before the Insurance Act scheme for spending 
vast sums of money on sanatoria was formulated. 
It is not too much to say that Prof. Pearson’s 
work must revolutionise our ideas on the subject. 
Briefly stated the results go to show that, at 
present, the influence of infection in the actual 
spread of the disease is small. The infection is 
so wide-spread that practically all the urban popu- 
