182 The American Naturalist. [February, 
anterior pole, both grow into the yolk mass towards the centre of the 
egg. The proctodeum is a narrow, straight tube, which finally reaches 
the anterior portion of the body and comes into contact with the stomo- 
deum. The latter begins in a short, straight esophageal portion at the 
base of the mandibles, which is distended at its central end into a vesi- 
cle resting against the blind end of the proctodeum. 
The most striking changes during this period are, however, found in 
the mesentoderm. It becomes differentiated into two layers, mesoderm 
and endoderm. The mesentoderm (or protendoderm) has, as has been 
seen, collected especially in the anterior end of the body, on either side 
of the median line. In the rest of the body it formed a layer of scat- 
tered cells in the surface of the yolk under the ectoderm. The lateral 
anterior masses become slowly marked off into a dorsal and a ventral 
portion. The dorsal portion forms a plate of cells on either side of the 
median plane in the anterior region of the body. The ventral part 
forms two ridges of cells (one on either side of the ventral mid-line), 
which are numerous, closely packed, and run down into the appendages. 
At the base of each pair of appendages there is a collection of these 
cells, to become muscles of the limbs. This gives the mesodermic 
ridges a metameric appearance. The two lateral dorsal plates become 
more and more clearly defined and enlarged. They are concave toward 
the yolk mass enclosed between them, and as they grow and meet on 
the mid-line, they unite. In this way a mass of yolk in the centre of the 
embryo is gradually included in a layer of cells, endoderm, originating 
from the mesentoderm. Since, however, the union of the endodermic 
plates is gradual, and since they meet first anteriorly and ventrally, the 
enteric vesicle formed by them is open behind and above for some 
time. The endoderm is now distinctly marked off from the rest of the 
mesentoderm, and has arisen from anterior lateral collections of this 
layer and from its dorsal portion. The rest of the mesentoderm be- 
comes mesenchymatous mesoderm. It is fuund everywhere beneath the 
ectoderm, especially ventrally. One other thing is more and more evi- 
dent in this region, where the mesoderm is collected most : the yolk has 
been absorbed gradually, and the mesenchymatous elements are here 
bathed in plasma. This absorption of yolk will continue rapidly from 
now on, proceeding from the ventral toward the dorsal surface of the 
embryo. 
During the last period of development profound changes take place, 
ending in the fully-formed young crustacean ready to leave the egg- 
membrane. The ectoderm soon secretes a cuticle, while the appendages 
elongate and become segmented. Those belonging to the head draw 
