542 TRANSACTIONS OF SECTION D. 
duration of the period of conjugation in Infusoria (Heterokaryota), (6) the cases 
of fertilised enucleate eggs that produce larvee with maternal characters. 
The evidence derived from the development of parthenogenetic eggs is 
contradictory, but does not, on the whole, support the theory that the chromo- 
somes are the only physical basis of hereditary characters. 
12. Pycnogonida (Sea Spiders)... By T. V. Hopason. 
13. On some Points in the Development of Ophiothria fragilis. 
By Professor E. W. MacBripg, I.4., F.B.S. 
Ophiothria fragilis is a common British ophiurid found in swarms near 
Plymouth, where the material for the researches recorded in this paper was 
obtained. ‘The egas are small and opaque, and cultures were obtained both by 
fertilising the egg artificially and by allowing males and females to spawn natu- 
rally. In addition a large quantity of material was obtained from the plankton, 
and by the use of all three sources it was found possible to make a complete study 
of the development, a full account of which will shortly be published in the 
‘Quarterly Journal for Microscopical Science.’ 
When the eggs are fertilised artificially segmentation results in the formation 
of a morula. An invagination of the outer layer of this follows, and it then 
transpires that the interior cells are precociously formed mesenchyme. The 
invagination of the outer layer takes place in such a way that a solid tongue of 
cells is left projecting into the archenteron. The ccelom then appears as a single 
vesicle at the apex of the gut, whilst the larva assumes a triangular shape, the 
two lateral points being the first rudiments of the larval arms. The primary 
mesenchyme migrates into these, and forms the basis of the calcareous rods which 
support the arms. 
When the eggs are fertilised naturally segmentation results in the formation 
of a thick-walled blastula; then a regular invagination follows without the for- 
mation of any tongue of cells. At the opposite pole of the larva from the 
blastopore a great crest of vacuolated cells is formed, which is absent in larve 
resulting from eggs fertilised artificially. The larva now assumes a triangular 
form, the coelom appears at the apex of the archenteron as a bilobed vesicle, and 
the crest slowly diminishes in size. 
The difference between these two kinds of development is to be attributed to 
the fact that the artificially fertilised eggs were not quite ripe, and hence were of 
different chemical constitution from ripeeggs. In certain features of their develop- 
ment the artificially fertilised eggs resemble the eggs of Ophiura brevis, a species 
with shortened development. Are mutations due to slight chemical differences in 
eggs at the moment of fertilisation ? 
In the subsequent history of the larva the single coelomic vesicle divides into 
right and left halves. Of these, first the left and then the right divide into 
anterior and posterior portions. Somewhat later from the anterior portions of 
both right and left sides are budded off sacs. That on the left side becomes five- 
lobed, and is the rudiment of the water-vascular system ; it subsequently grows 
round the cesophagus. That on the right side, the equivalent of the dorsal sac 
of the Echinopluteus larva and of the Bipinnaria, ordinarily remains small, but 
sometimes assumes a five-lobed form, showing that it is a rudimentary fellow of 
the water-vascular system. The larva of Ophiothria fragilis affords final and 
convincing proof that the echinoderm larva possesses three somites, the middle 
one of which becomes on the left the water-vascular system. 
14. The Rise and Recognition of Economic Biology. 
By Water E. Couuince. 
