632 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
it is conical. A cell-membrane completely surrounds the spermatozoon. 
The tail is in connection with the cell-membrane, and not attached 
directly to the nucleus or to the middle piece. The number of sperma- 
tozoa formed from a spermatogone is four, by means of two mitotic 
divisions. Each of these divisions is a reducing division. The number 
of chromosomes in the spermatozoa is nine. This number is charac- 
teristic for Echinoderms. When the nucleus of a spermatozoon in the 
process of fertilisation has passed the outer denser cytoplasmic portion 
of the ovum it increases in size. The small refringent body seen by 
various investigators at the apex of the head of the spermatozoon is the 
centrosome. It is extranuclear in position in the spermatid and sper- 
matozoon, possibly intranuclear in the spermatogone and spermatocyte. 
It seems probable that the centrosome and mitrosome are differentiations 
of one and the same substance ; they represent the material of the 
nuclear spindle. The differentiation of these bodies may be a device for 
overcoming the mechanical difficulties of transferring a large quantity 
of spindle-forming substance through the egg-membranes and denser 
outer portion of the egg. 
Development of Echinoderm Larvae.* — Mr. H. M. Vernon has made 
a study of the effect of environment on the development of Echinoderm 
larvae. He finds that, if the ova of Strongylocentrotus lividus be placed 
in water at about 8° or 25° for an hour, or even for a minute, at the time 
of impregnation, the resulting plutei, after eight days’ development, are 
some 4*4 per cent, smaller than those of ova impregnated at from 17° to 
22°. If kept at the abnormal temperature for only ten seconds during 
impregnation, the resulting larvae are only 1 • 7 per cent, smaller, pro- 
bably because the time is too short for all the ova to become impregnated 
under the abnormal conditions. Larvae obtained from artificial fertilisa- 
tions made in the middle of August are about 20 per cent, smaller than 
those obtained in April, March, and October, while those obtained in Juno 
and July are intermediate in size. This is probably due to the com- 
parative immaturity of the ova and spermatozoa in the off-breeding 
season. Larvas allowed to develop in water containing 50 ccm. of distilled! 
water per litre are 15 * 6 per cent, larger than those grown under normal 
conditions. Larvae developed in water more concentrated than the normal 
remain practically unchanged, but those grown under normal condition^ 
from ova impregnated in concentrated water, are 1 • 6 per cent, larger. 
Larvae grown in semi- darkness are 2*5 per cent, larger than those grown 
under normal conditions, while those grown in absolute darkness are 
1 • 3 per cent, smaller. The body -lengths of the larvae are not influenced 
by the number of larvae developing together in a given quantity of water, 
if they be kept below 30,000 per litre. Larvae grown in water contain- 
ing an additional amount of carbonic acid gas are slightly larger than 
the normal. On an average, the aboral and oral arms of larvae grown 
in water containing 4000 larvae per litre are, respectively, 13*4 and 15*9 
per cent, shorter than those in water containing 500 per litre. The body- 
lengths of larvae developed in diluted sea- water is, on an average, in- 
creased by 9 * 1 per cent, while the arm-lengths are decreased by 7 • 7 per 
cent, and 10*5 per cent. Hence, as the arm-lengths are percentages of 
the body-lengths, the absolute arm-lengths are not affected at all. The 
* Phil. Trans., B. 186 (1896) pp. 577-632. 
