SUDLER : DEVELOPMENT OF PENILIA. 
119 
tral surface is farther away from the mid-line and more toward the 
side. Thus it is possible to distinguish almost exactly the com¬ 
parative importance of the two mechanical factors most concerned 
in the revolution. 
The embryos now increase in length so that the posterior end of 
each embryo lies still further back and to the outside of the anterior 
end of the embryo just behind it. The height of the brood-chamber 
increases, and still further growth and flattening of the embryo 
caused by the enlargement of the second antennae tend to raise 
the anterior of each embryo above the intestine and allow the 
smaller rounded end to drop so that it now lies to the outside and 
beneath the anterior end of the embryo immediately behind it. 
This constantly increasing factor adds to the rotation until it may 
cause the side which was originally next to the outer wall to lie on 
top of the intestine. With the increase in size in all directions the 
embryos become very much crowded in comparison with their condi¬ 
tion at first. The brood-chamber also enlarges, and the floor is 
raised until it is on a level with the intestine. A cross-section of it 
now shows it to be hemispherical in shape rather than heart-shaped. 
The embryos now lie almost wholly above the intestine and have 
lost to some extent the slant in the dorso-ventral jflane. The 
crowding occasioned upon the maturity of the embryos causes many 
changes of an irregular nature in their positions, so that they depart 
from the positions described, especially in regard to the ventral 
side being uppermost or directly dorsal. In nearly mature embryos, 
however, their long axes are more nearly parallel with that of the 
mother than before. The position of the embryos of Penilia is thus 
seen to be a changing one in the different stages of their develop¬ 
ment. To a certain extent, the degree to which the embryos are 
shifted depends upon their age, but throughout their development 
the anterior end of the embryo lies toward the corresponding end 
of the mother. In older stages, the ventral side is uppermost and 
looks toward the side, except as noted, where it is just the opposite, 
looking downward and toward the intestine of the mother. While 
the axis of the embryo is never exactly parallel to that of the 
mother, in a general way it always lies in the same direction, 
sufficiently so to see with jjerfect clearness that the egg is oriented 
and that as it develops its position in regard to the mother is 
changed by a series of mechanical factors resulting from the 
growth and the enlarging brood-chamber to accommodate its 
increasing contents. 
