RELATIONS OF YOLK TO GASTRULA IN TELEOSTEANS. 5 
G. merlangus . 5 . 1:25 mm. diameter. 
CrAmlOn CMAs Mieka, Riana lca)», a5 ks bag 
G. eglefinus . : BP eae » 
Trigla gurnardus. aod st ees 9 
Clupea harengus . See hay 99 
The measurements of the first four were obtained by mea- 
suring the drawings of them made by means of Abbé’s camera 
lucida, always with the same combination of lenses in the 
microscope. The measurements so obtained are, perhaps, not 
absolutely accurate, but the errors are about the same in all 
cases. The ovum suffers probably some slight alteration in 
shape, due to its own weight, when resting on a glass slide in a 
small quantity of water, which were the conditions under which 
the drawings were made. The ova, too, of one species, vary 
slightly in size; but the above measurements are at least 
approximately correct. The diameter of the herring egg given 
is taken from Kupffer. 
The fertilised ovum of each of the three species of Gadus 
mentioned consists, soon after fertilisation, of a sphere of per- 
fectly transparent, structureless, colourless yolk, at one pole of 
which is a mass of protoplasm forming the blastodisc. The 
blastodise projects slightly beyond the regular contour of the 
sphere, and has a peculiar colour, resembling a light shade of 
what is known as terra-cotta colour. This colour remains during 
the process of simple segmentation, but it gradually gets lighter, 
and disappears at the time when the segmentation cavity is 
formed. After this period the protoplasm of theembryo is colour- 
less, and only a little less transparent than the yolk mass. The 
ovum is enclosed in a vitelline membrane, which is thicker in 
the cod than in the other species, and in which no structure is 
to be observed with a power of forty diameters. Between the 
ovum and the vitelline membrane is a space filled with homo- 
geneous fluid, chiefly, no doubt, sea water. This “ perivitel- 
line” space remains throughout the whole development without 
varying in size. In the ova of Gadus it is not very large, but 
I have seen other ova taken in the tow-nets in which the dia- 
meter of the perivitelline space at its widest part was equal to 
