ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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variability of tbe larvae in respect of the body-length steadily declines 
as the period of development proceeds from the fifth day. The variability 
reaches a maximum at 18° to 20°, the temperature most favourable for de- 
velopment. These, we may add, are some only of the interesting conclu- 
sions to which Mr. Yernon was brought by his laborious experiments. 
The conclusions arrived at have some bearing upon the problems of 
variation and natural selection. The larvte evidently showed a consi- 
derable amount of variation in size, quite apart from any influences 
caused by conditions of environment. This variation may be consider- 
ably increased by the operation of changes in the environment during 
development. Other things being equal, the greater the variation in any 
group of organisms, the greater chance have the organisms as a whole of 
becoming modified by natural selection. As probably no organisms are 
as perfectly adapted to their surroundings as it is possible for them to 
be, it is important to them that they should be as variable as possible, so 
as, by the action of natural selection, to become still better adapted. 
Though it has not been conclusively proved that the ovum is specially 
sensitive at the time of impregnation to other conditions than that of 
temperature, yet the probability is that this is the case, and other con- 
ditions, such as the salinity of the water, also have a more powerful 
influence. Changes of environment were found to produce different and 
opposite effects upon different parts of the same organism. For example, 
as we have already seen, a fall in the temperature, or a decrease in the 
salinity of the water, produced a decrease in the arm-length of the larvae 
and an increase in the body-length. It is thus possible, Mr. Yernon 
points out, for parts of an organism to become modified, though they 
may be entirely unoperated upon by the action of natural selection, or 
may even serve some useful purpose to the animal. If, for instance, it 
is of greater utility to the larvae that their body -lengths should increase 
rather than their arm-lengths, and if, by the operation of a fall in the 
temperature of the water, larvae with greater body-lengths and smaller 
arm-lengths are produced than on an average, the larvae exhibiting these 
characteristics of the most marked extent will survive, and the race will 
be modified in that direction. 
Egg of Sea Urchin.* — Prof. E. B. Wilson has made a study of the 
archoplasm, centrosome, and chromatin in the egg of Toxopneustes. The 
general results of his observations may be thus summed up. The sperm- 
aster arises by the morphological rearrangement of the general cyto- 
reticulum under the influence of a central mass derived from tbe middle 
piece of the spermatozoon. The astral rays arise by the linear arrange- 
ment and fusion or close union of the granules of the reticulum. The 
spindle-fibres are entirely formed within the nucleus. At the close of 
karyokinesis the spindle-fibres break up into granules. The asters 
persist after cell-division, and finally themselves divide to form the 
daughter-asters, which persist through the ensuing “ resting stage.” The 
amount of chromatin largely increases during the pause which follows 
the fusion of the nuclei. At the close of the pause a large part of the 
chromatin appears to be converted into linin, and from this the spindle- 
fibres are largely derived. The staining power of chromatin is at a 
minimum immediately after the reconstruction of the daughter-nuclei. 
* Journ. Morphol., xi. (1895) pp. 443-78 (12 phototypes). 
