TRANSACTIONS OF SECTION D. 1095 
mass. Professor McIntosh has pointed out that the mussel reaches full re- 
productive maturity in April, and that there is a gradual disappearance of the ova 
or spermatozoa during June and July, until, in the latter month, most examples are 
quite empty. The female generative organs are almost invariably redder than the 
male, and the groups of sperm-sacs are more prominent than the ovigerous masses. 
When an incision is made in the fully ripe male or female organ, a creamy fluid 
issues, holding respectively immense numbers of spermatozoa or ova. In the ovary 
the ova are surrounded by a transparent hyaline investment, and, when fully 
ripe, the germinal area and germinal spot are lost to view, the vitellus becoming 
densely granular. The genital canal is readily seen, and microscopic sections show 
it to be clothed internally with ciliated epithelium. Natural fertilisation in all 
likelihood takes place in the surrounding water. Artificial fertilisation is easily 
accomplished. A piece of tissue containing spermatozoa is minced in a watch-glass 
with sea-water, and a little of the milky liquid decanted into another glass. Into 
the latter is then poured a little liquid containing ova procured in the same way. 
The glass is kept cool, and in half an hour the milkiness is removed from the liquid 
by repeated washings, and particles of débris sucked up by a fine pipette. Hach 
ovum has now a considerable number of spermatozoa attached to it, and their 
wriggling causes it to rotate. In about four hours the clear polar or direction-cell 
appears. Decreasing vigour and changes of temperature cause conflicting results as 
to the time certain developmental stages are reached. Fertilisation so late as 
August 1 was partially successful. The writer's earliest attempt (and it was the 
most successful) was made in the beginning of June. The first segmentation takes 
place immediately after the appearance of the polar cell, resulting in a larger 
segment (macromere) and a smaller (micromere). Repeated budding of the micro- 
mere takes place, and in six or seven hours the embryo assumes its most irregular 
form. Thereafter it is gradually reduced to a more regular shape, and in ten hours 
the brownish granular contents have disappeared. In fifteen or eighteen hours the 
shape is almost spherical, and the contour is broken by the projecting part of what 
is probably the still undivided macromere. The free edge of this body is crenate. 
‘The existence of a central cavity can now be made out, the polar cell still persists 
in many cases, and the embryo rotates by means of minute cilia which seem to 
cover the greater part of its surface. Two examples (of distinct series), forty-three 
hours old, were characterised by having, besides the minute cilia, in the one case 
one strong cilium at least as long as the diameter of the embryo ; in the other by 
two cilia somewhat longer. In both forms there were features indicating approach- 
ing differentiation of structure. Embryos have been kept alive for four days, but 
not in a healthy condition. 
The very youngest forms taken by the tow-net from the surface of St. Andrews 
Bay, where, in July and August, they abound in great numbers, have the body 
wholly enveloped in a transparent shell, and the various organs are still very 
rudimentary. By the time they sink to the bottom, the foot, byssus gland, 
revolying otoliths, liver, and gill-papille are well seen. The foot, which is 
ciliated at the tip, adhesive, and extremely extensile, is the means ‘of active 
locomotion. Amongst the smallest examples of a previous season, many not more 
than one-eighth of an inch in length contained in their tissue either ova or 
spermatozoa, presenting no appreciable difference from those of the adults. They 
were probably not more than a year old. 
6. On the Modification of the Trochal Dise of the Rotifera. 
By Professor A. G. Bournz, D.Sc., F.L.S. 
It is now a generally accepted theory that this structure is the homologue of 
the ciliated bands of the larve of Echinoderms, Chetopods, Molluscs, &c., and of 
the tentaculiferous apparatus of Polyzoa and Gephyrea, and is often termed in 
common with these a ‘velum,’ This velum presents itself in various stages of 
complexity. It is found as a single cireum-oral ring (Pildiwm), as a single pree- 
oral ring (Cheetopod laryz), or as a single pre-oral ring, co-existing with one or 
