36 Edi. 
ICCHlNODEiiMATA. 
which is placed on the side opposite to that from which the polar globules 
arose. After fusion of the male and fomalo pronuclei, tlio ovocentru 
and spermocentre are on opposite sides of the first segmentation nucleus. 
They next proceed each to divide in two, and tho halves travel away 
from one another round the nuclous, each passing through a right angle. 
As a result of this quadrille-like march, the demi-ovocentres come to 
meet the demi-spermocentres, and unite with them to form the first 
“ astrocentres,” while the first amphiaster is forming. 
The author concludes that fertilization consists not only in the fusion 
of two demi-nuclei from different sexes, but also in the union of two 
demi-spermoceutres with two demi-ovocentres to form the first two 
astrocentres, and all astrocentres of the offspring, being derived by divi- 
sion from the two primitive astrocentres, originate equally from the 
father and from the mother. 
Jaekel (2) discusses some young stages of HolopocrhiicUu. The life 
conditions under which the family live [cf. Physiology and Biology, p. 43] 
make it absolutely necessary that the larvae also should be very strongly 
and massively built, iu order to be able to maintain themselves on the 
spot where the colony is fixed. Hence the young forms show only 
ccenogenetic adaptations. • 
Ludwig (3) describes the development of Cucumaria planet from the 
eighth day onwards. The plane of symmetry of the young Echinoderm 
does not coincide with that of the larva; but iu the anterior (oral) region 
of the stage transitional between the larva and the young animal, the 
plane of symmetry diverges from that of the larva towards the left, and 
in the posterior region towards the right. The two planes of symmetry, 
therefore, cut one another at acute angles. The longitudinal axis also of 
the young Cucumaria does not coincide with that of the larva, but 
diverges towards the ventral surface in the anterior region, and towards 
the dorsal surface in the posterior region. The water- vascular ring and 
radial canals have assumed their permanent position on the eighth day. 
The median ventral vessel has the two first tube feet, and on the follow- 
ing days exceeds in length and diameter the other four vessels, of which 
also the latero- ventral vessels arc shorter and narrower than the two 
latero-dorsal. Muscle-fibres appear first in the median ventral vessel, 
then in the latero-dorsal, and lastly in the latero-ventral. The muscle- 
fibres are all longitudinal, and are formed from the cells of the epithe- 
lium of the hydrocoele. Five tentacles are developed on the eighth day, 
which lie in an oral atrium, into which they can be retracted. They are 
all situated in front of the second ciliated band of the larva (regarding 
the cilia of the cephalic hump as the first band). The tentacles are 
arranged asymmetrically with regard to the larval plane of symmetry, 
but symmetrically with regard to that of the adult. In front lies an 
unpaired tentacle, followed by four others in two pairs ; the exact 
opposite to the arrangement described by Selenka. The tentacular 
vessels arise from the growing radial vessels, and are not arranged in 
regular radial fashion, but are asymmetrical. The two tentacles of the 
