122 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Tiie Maxillae. 
A maxillary region is differentiated immediately after the man¬ 
dibles are well formed, and upon this the appendages appear later. 
The first pair of maxillae appear first and the second pair follow 
shortly. The first are the largest and show themselves as two 
small rounded eminences projecting at first away from the middle 
line but closer to it than the mandibles. Their position as devel¬ 
opment proceeds changes but little, except that the appendages of 
the adult point more toward the middle line. They are small even 
in the adult and consist of but a single joint bearing setae on its 
terminal extremity. The second maxillae develop at first as small 
thickenings which increase until the appendages are small projec¬ 
tions, reaching their maximum development in a stage represented 
by Fig. 27, when the thoracic appendages are just beginning to show 
the first trace of bifurcation. These appendages disappear as rapidly 
as they come, and by the time the bifurcation of the thoracic append¬ 
ages is well marked they have become reduced to mere darker stain¬ 
ing masses which soon cease to be recognizable as appendages. The 
second maxillae are formed sooner in Penilia than in Moina, where 
they appear after the thoracic appendages have already bifurcated. 
They persist, however, much longer in Moina, disappearing only just 
before the larvae are set free. They are much more transient in 
Penilia and never so well marked. 
The Thoracic Appendages. 
The thoracic appendages follow the usual rule and appear in a 
regular sequence of order making the anterior the oldest and the 
posterior the youngest. The thoracic region becomes divided into 
six segments in the same order. Furrows running from the ventral 
side up to the dorsal side appear. These cause the prominences 
between them to become more pronounced as development proceeds. 
This condition causes the peculiar fluted appearance presented by the 
embryo in side view as seen in Fig. 26. These prominences approach 
much closer to the middle line on the dorsal than on the ventral side. 
A view of the embryo at this stage shows the prominences projecting 
slightly beyond the ventral surface and squarish in outline. This 
