ON DEVELOPMENT OF ATYEPHIRA COMPRESSA. 
413 
lar and forms the first rudiments of the carapace (figs. 54, 55, 
56, cp.). On the side diametrically opposite to this two oval 
elevations (figs. 56, 58, 59, oc.) appear. These are at first 
somewhat separated from each other, but their interval is 
gradually lessened until they become connected together bv an 
elevation of the intervening space. These are the first traces 
of the cephalic lobes. These gradually travel upwards (or mor- 
phologically backwards) as will be seen from figs. 60 and 61, 
oc. Immediately behind these the first traces of the first pair 
of antennae become visible as oval elevations, and a little 
smaller than the cephalic lobes (fig. 60, At. 1). Then the 
second pair of antennae (fig. 60, At. 2) is formed, so that the 
order of the formation of the parts of the embryo is as 
follows : — Abdomen, mandible, cephalic lobes, carapace, the 
first pair of antennae, and the second pair of antennae. 
After a while the cephalic lobes come closer together, the 
first pair of antennae elongates, and a crescent-shaped depression 
(fig. 61, lb.) is produced on the median line of the embryo in 
the region between the first pair of antennae, which marks out 
the labrum. The second pair of antennae, which up to this 
time was single, now becomes bilobed (fig. 61, At. 2), and the 
abdomen (fig. 61, ab.) takes a more spherical form. A section 
at this stage (fig. 62, ab.) shows that the cells of the abdominal 
region are much larger than those of other parts. The meso- 
dermic cells (fig. 62, ms.) are very much crowded in the tho- 
racic region, and a few in the cephalic and abdominal. This 
is the nauplius stage of the embryo. 
In Palaemonites vulgaris, according to Faxon ( S -£J), 
“ the parts of the embryo which first appear are the abdomen, 
the labrum, and the cephalic discs, and the first three pairs of 
appendages.” The rudimentary carapace, which in Atye- 
phira compressa appears before the first three pairs of 
appendages are formed, here comes into view after the third 
pair of maxillipedes is formed ( 3 1 ^ 8 ). 
Stomodseum and Proctodseum. — Soon after the closure 
of the gastrula cavity a fresh invagination of the ectoderm 
takes place slightly in front of the late blastopore. This gives 
