MUSEUM OF COMPARATIVE ZOOLOGY. 75 
alike in constitution ; some of them are stained deeply and appear homo- 
geneous, while others are stained lighter and appear granular. During 
the whole of this period there continue to remain in the yolk mass a 
large number of cells, which are distributed through its substance at 
tolerably regular intervals. There is often a comparatively small amount 
of protoplasm enveloping the large angular nuclei of these yolk-cells, and 
about them the yolk corpuscles are more or less definitely grouped. 
3. At the beginning of the third period the embryo still has a trans- 
versely banded appearance as in the protozonite stage ; the concentration 
from the sides is completed, and about six zonites are distinguishable 
between the head- and tail-lobes. The zonites now begin to grow thin- 
ner in the ventral median line, and at the same time their ends become 
gradually more prominent and rounded. The small knob-like promi- 
nences at the ends of the zonites are the rudiments of the appendages, and 
in about two days after their first appearance (at the temperature stated) 
the six cephalo-thoracic appendages are fully established as represented 
in Pl. Il. fig. 7. The two anterior pairs of appendages are much 
smaller than the four succeeding pairs, the latter being about equal in 
size. The appendages thus established correspond to the chelicere, the 
pedipalpi, and the four pairs of ambulatory appendages of the adult. 
Simultaneous with the growth of the appendages new zonites, derived 
from the tail-lobe, make their first appearance; the four anterior of 
these are very prominent, and a little later they bear four pairs of pro- 
visional appendages (Pl. IV. fig. 20, pr. app.). In this first part of the 
third period the head plate is faintly bilobed; the tail-lobe is broad and 
rounded. 
A ventral view (Pl. IV. fig. 19) of the same ege (Pl II. fig."'7) 
shows a faint median furrow, which marks the thinning out of the ecto- 
derm in the median plane after the separation of the lateral halves of the 
underlying mesoderm. There are slight elevations just inside the bases 
of the limbs, best seen in optical section along the upper margin of the 
figure; they are the beginnings of the nervous ganglia. 
At first the appendages grow out perpendicular to the axis of the 
body (Pl. II. fig. 7), but as they increase in length they curve towards 
the median line, as shown in Fig. 8. They are now indistinctly four- 
jointed. The central lumen, which can be observed readily in optical 
sections of the leg, is shown by actual sections to be a prolongation of 
the cavity of the corresponding mesodermic somite. 
At the present stage —the last part of the third period —the head 
plate has become distinctly bilobed, a prominent upper lip composed of 
