168 Proceedings of the Royal Society 
intensity of tlie agitation. Like most pelagic eggs, those under 
consideration are transparent, and each egg is separate and spherical. 
The eggs of the whiting are the smallest of the three species, the 
cod’s next in size, and the haddock’s largest. The diameter of the 
latter measures 1‘45 mm. Owing to their convenient size and 
perfect transparency, the changes which occur in the embryonic 
period of development — that is, from fertilisation to hatching — can 
be followed with great ease by placing eggs at successive stages under 
the microscope, a combination of lenses being used magnifying about 
40 diameters. The eggs were obtained and fertilised by squeezing 
them from the ripe fish on board a fishing boat into a bottle of sea 
water, into which milt from a ripe male had previously been allowed 
to fall. 
The eggs of all the species mentioned have a very similar structure ; 
the yolk is perfectly structureless, the vitelline membrane very thin, 
and the pervitelline space of moderate size. At one pole of the yolk 
is the blastodisc, a cap of protoplasm tinged at the earlier stages of 
development of a light terra-cotta colour. This colour gradually 
disappears towards the end of the process of simple segmentation. 
The processes of the development in pelagic Teleostean ova have 
been recently described by various naturalists, and there is a general 
agreement in the descriptions concerning the main features. Full 
details of my own observations will appear shortly in the Quarterly 
Journal of Microscopical Science. All I wish to do here is to 
review synthetically certain phenomena of the development, their 
probable significance as evidences of remote ancestral conditions, 
and their homological relations to phenomena in the development 
of other vertebrate forms. 
The development of a pelagic Teleostean ovum, and doubtless of 
all Teleostean ova, differs from that of all other vertebrate ova in the 
fact that invagination takes place round the whole periphery of the 
blastoderm simultaneously. This invagination is completed before 
the blastoderm begins to grow over the yolk mass, and the inva- 
ginated layer causes the peripheral region of the blastoderm to be 
thicker than the central part. The thickened part is known as the 
germinal ring. Along one radius the blastoderm is thicker than 
elsewhere, even before the inflected layer appears, and beneath this 
radius the inflected layer when it is formed extends further inwards 
