118 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
gastrula mouth and making the embryo bilaterally symmetrical. 
The first pair of these prominences become the first or sensory 
antennae, while the second will be the chief locomotor organ of the 
adult, the second or swimming antennae. 
Up to this time the position of the embryos has been practically 
the same and only a very slight change has occurred from that 
first noticed in the segmenting eggs. The embryos have elongated, 
and that is all. As the appendages appear the change in shape of 
the embryos and of the enlarging brood-chamber causes a shifting 
in jmsition of the embryo with reference to the mother. 
Upon looking at the cross-section of an adult with a brood- 
chamber as previously described it is seen to be heart-shaped with 
the apex upwards at the dorsal wall and the intestine occupying 
the depression in the blunt end of the heart. 
Before any change in the outward shape of the embryos occurs 
they lie for the most part in the concavity on either side of the 
intestine with the posterior ends of the ones in front to the outside 
and somewhat beneath the anterior ends of the embryos directly 
behind. When the appendages push out the flattening at the 
anterior end occurs, and a resistance is encountered on either side 
by growing against the outer walls of the brood-chamber externally 
and against the wall of the intestine internally. The outer an¬ 
tennae strike the wall at an angle so that an increase in width 
causes the edge of the embryo to rise and revolve slightly on its 
other antenna, as an axis, in such a way as to cause the region 
once occivpied by the gastrula mouth to look dorsally and outward 
instead of outward with a slight ventral tendency. An exception 
to this sometimes occurs in the case of the most anterior embryos 
which do not have the other factor; i. e. the posterior end of 
another embryo under the outer side of its anterior end and chang¬ 
ing its position. These embryos in some cases may assume such a 
position that the angle formed by the antenna and the outer wall 
of the animal is in the opposite direction from the rest. This 
causes a rotation in the other direction so the position once occu¬ 
pied by the gastrula mouth is downward and in, instead of upward 
and out, as in the rest of the embryos. This opposite rotation 
seems to affect only the most anterior embryo and even in this 
case it is the exception and not the rule. The normal or usual 
position assumed by these anterior embryos is not exactly like that 
taken by the rest in the brood-chamber but one in which the ven- 
