1915] On Ciupea PALLAst1 CUVIER AND VALENCIENNES 105 
germinal disc is distinctly visible, and in less than half an hour more 
the first division is completely through the disc. In three hours division 
has again taken place, from which times it proceeds regularly until by 
the end of 8 hours the mass has arrived at approximately the 32-celled 
stage. From this on the number of cells increases rapidly, so much so 
that at the end of 12 hours almost one-fourth of the surface of the 
yolk is covered with the cellular disc, which is not very distinct from 
the yolk at its margin. In 24 hours the margin of the disc is more 
distinct and is showing somewhat thicker towards the centre, the boun- 
dary between the cell-mass and the yolk below being discernible in a 
side view. In 36 hours a distinct ridge is noticeable extending from 
edge to edge of the cell-mass which now covers about half of the yolk. 
This gradually increases in length as the mass extends over the yolk, 
but shows no definite thickening at any one part until after the 6oth 
hour, although it thins out at the ends. By this time it has reached 
fully three-fourths of the way around the yolk. Soon after this the end, 
which later becomes the head, begins to thicken and become rounded 
off so that by the end of the third day the ridge begins to take a definite 
shape. During the next 12 hours much change takes place. The head 
becomes well defined with the eye showing, the brain and spinal cord 
are differentiated and several somites make their appearance, but the 
tail is still indistinct. The embryo reaches nearly nine-tenths of the 
distance around the yolk. At the end of four days the tail has become 
differentiated, the somites have become quite numerous, while the head 
is still further developed. In 4} days the ear has made its appearance, 
some indication of a heart is present, and the division into somites has 
taken place pretty well throughout the length of the body. The embryo 
has increased in length so that the tail now touches the tip of the nose, 
a small portion of the tip being free from the yolk. During the fifth day 
the head and a large portion of the tail becomes free from the yolk and 
the body is twisted slightly so that the tip of the tail goes past the head. 
The heart takes more definite shape and can be seen to beat. By the 
end of the fifth day the embryo itself can move about and from that 
time on shows considerable activity in rolling about in the limited 
space at its disposal. From the time that bodily movement first takes 
place until the embryo is hatched out the change is very gradual, con- 
sisting largely of the growth and development of the organs already 
present. No definite time can be given for the appearance of other 
organs, as the rate of growth differs materially in different cases, with 
a corresponding difference in time for hatching out. Under normal 
conditions the shortest time required for hatching was 123 days and 
the longest 22 days. Much greater numbers come out in from 14 to 
