SUDLER : DEVELOPMENT OF PENILIA. 
121 
The Second Antennae. 
One of the first changes noticed in the outward shape of the 
embryo is a widening of the anterior third and the appearance of 
two rounded prominences projecting posteriorly on either side of 
this area which will form the second antennae of the adult. These 
two projections grow backward rapidly and, at a stage when the 
maxillary region and two thoracic segments are present, bifurcate. 
Their position in regard to the entire animal shifts, approaching 
more and more the anterior end of the embryo as the head bends 
and in the adult coming to occupy the extreme anterior dorsal 
corner of the animal. From their first appearance the second an¬ 
tennae are the most prominent appendages. They grow rapidly 
and appear first and thus foreshadow their great importance in the 
adult. The swimming hairs do not begin to show until just before 
the embryos are mature. The second antennae are folded back 
close to the side of the embryo as long as they are in the brood- 
chamber of the mother. 
The Mandibles. 
The mandibles are the next appendages to appear and the time 
when they are first recognizable varies but little. At a stage 
represented by Fig. 21, when the second antennae are well 
defined as two processes projecting posteriorly, the mandibles show 
as paired outgrowths just posterior and ventral to the second 
antennae. These eminences do not project either posteriorly or 
anteriorly but directly outward from the middle line. As the 
embryo grows older they assume an upright position (project ven- 
trally) and approach more and more the middle line. In the adult 
they meet in the middle line. They are stout, serrated on their 
distal borders, and moved by well-developed muscles. (Fig. 1.) 
Except for the change in position in the direction in which the 
appendages point, there is no shifting such as one sees in the case 
of the two previous appendages described. Their position in the 
adult is practically that occupied by them throughout the embryonic 
development. 
