PROCEEDINGS OF SECTION D. 485 
In the meantime the blastodise has become (after the 32-cell 
stage) two layers thick in its central portion. It lies directly on 
a thin layer of non-nucleated periblastic material, which extends 
round the whole vitellus. There is at this stage no trace of a 
segmentation cavity. The blastoderm now spreads out as a very 
thin layer over the ectodermal extremity of the egg. The thick 
blastoderm of 16 cells becomes converted in eighteen hours into a 
thin cap of very numerous small cells, covering about a quarter of 
the yolk. This cap is at first perfectly uniform, but soon changes 
appear, by which it is marked out into an embryonic (posterior) 
and a non-embryonic (anterior) portion. 
The first of those changes, which becomes marked when the 
blastoderm extends over about a third of the yolk, is the forma- 
tion of a thickened rim, having the appearance of being produced 
by a bending inwards of the edge. About this time also a cavity 
(segmentation cavity) appears underneath the anterior (non- 
embryonic) part of the blastoderm. This cavity has a very short 
duration, soon becoming obliterated. It intervenes between the 
blastoderm proper and a thin layer of periblast, with scattered 
nuclei, which forms its floor. A depression then appears just 
within the anterior border of the ring, bounded behind by two 
rounded elevations. The depression, which is of small extent, is 
the non-embryonic part of the blastoderm, the rest, ending in front 
in two convexities separated by a median notch, is the embryonic 
shield. As the blastoderm extends further over the yolk, both 
the embryonic and the non-embryonic portions are increased in 
size. The two convexities of the anterior border of the embryonic 
shield coalesce to form one median prominence, which marks the 
position of the anterior border of the head of the embryo. When 
the blastoderm has passed the equator of the egg an axial 
thickening, at first very narrow, appears, running from the 
posterior border to near the anterior margin of the embryonic 
shield. Its direction of growth seems to be from behind forwards, 
and it probably begins at the thickened posterior border, into 
which its posterior extremity passes out laterally. In some 
instances there is a slight break or notch in the thickened border 
of the blastoderm at the end of the axial thickening; but this 
does not seem to be of constant occurrence. 
When the blastoderm covers three-quarters of the surface 
of the egg the axial thickening has become somewhat 
broader and is growing downwards into the yolk as a keel-like 
ridge. This keel is much more strongly developed in its anterior 
half ; behind it decreases greatly in size. 
Sections through embryos with the keel in various stages of 
development show that epiblast and mesoblast are completely 
fused in the whole length of the keel. This is entirely at variance 
with what has been observed in other fishes. Goette, for example, 
states that in the trout there is no coalescence of the layers along 
