EMBRYOLOGY. 99 
pact mass several cells thick, some of "which are vacuolated, and 
having a coarsely granular nucleus. Figure 3 of Plate IV shows a 
cross section — slightly oblique, however — of the germ band. 
The yolk granules of the primary yolk are now more numerous 
near the embiyo. 
In Stage 5 (PL TV, fig. 4) the germ band has clearly differentiated 
into the amnion and the embryo proper; these gradually merge into 
each other. Tins differentiation apparently takes place by a gradual 
migration of cells to one side of the germ band. The cells of the 
amnion at tins time resemble very closely those of the germ band 
proper. The genu band begins to fold in this stage and its anterior 
extremity begins to broaden and flatten. The ovarian yolk has de- 
creased in volume and has assumed a more anterior position in rela- 
tion to the embryo. The yolk cells in both the primary and ovarian 
yolk have lost somewhat their amoeboid character, and now consist, 
each, of a large granular nucleus, with a much thinner area of pro- 
toplasm about it. The primary yolk granules are smaller and much 
less numerous than before and are collecting in masses about the 
yolk cells, with indications here and there of a partition, or wall, 
forming between them. This stage is reached by the end of the second 
day, under favorable weather conditions. 
The "polar organ'' and protoplasm at the posterior pole contain a 
large central vacuolar area now. 
In Stage 6 (PL V, fig. 1) the germ band has greatly increased in 
length, is folded upon itself, and almost forms a loop, the anterior 
and posterior extremities nearly touching, and both pointing to the 
posterior pole. A portion of the posterior extremity of the germ band 
is again folded upon itself. It is now differentiated into three layers, 
which we take to be, respectively, ectoderm, mesoderm, and ento- 
derm. The ectoderm and mesoderm consist of a compact mass of 
columnar cells, two cells thick. The entoderm is much thinner and 
less compact and forms an almost continuous sheet over the inner sur- 
face of the germ band. Its cells resemble yolk cells very closely. 
In this stage fundaments of the body segments appear as slight 
elevations of the ectodermal surface. The ovarian yolk has assumed 
a more anterior position in relation to the embryo than in the pre- 
ceding stage. Between the ovarian yolk mass and the germ band is a 
group of cells that have apparently separated off from the mesoderm. 
From this group of cells, in later stages, the generative organs arise. 
The amnion now r covers the ventral surface of the embryo and the 
other surface of the embryo is in contact with the yolk. The amnion 
is a very thin, delicate membrane, its cells being widely spaced and 
quite small. The intervening protoplasm between the cells of the 
serosa has become more constricted and the cells have taken more of 
an elongated oval shape. The primary yolk has now become defi- 
