124 PATTEN. [Vou. XII. 
determined with certainty. There is, however, little doubt in my mind that the 
two appendages in question are the fifth and sixth thoracic appendages of the left 
side. The sixth has been moved in the plane of the paper 45° toward the 
embryo’s right side. The cause of the rotation is to be sought, as in Fig. 53, in 
the absence of the posterior right half of the thorax. 
The mesodermic area is much contracted, and in place of the continuous 
thickened rim are isolated masses of closely packed nuclei, some of which lie 
quite deeply in the yolk; others are continuous with the surface mesoderm. The 
masses are irregular in shape, but are usually provided with radiating streamers 
containing many nuclei. 
Fic. 55, X 60, sectioned. This is also an hour-glass embryo, but one in which 
the two parts are completely separated. The cephalic lobes are absent. The 
second pair (chelicerae ?) of thoracic appendages have fused in the median line. 
The next pair are widely separated, and following them is a second pair of fused 
appendages. Between the unfused appendages is a well-developed double nerve- 
cord, which gradually narrows at either end to an unpaired cord. One end lies 
just back of the anterior median appendage and the other in front of the posterior 
one. 2 
Back of these four appendages is an area devoid of external features. In 
sections it is seen to be composed of a slightly thickened homogeneous layer 
of ectoderm with a similar underlying layer of mesoderm. At the posterior end 
of this area is a deep, tubular invagination or telopore, ¢.f., directed vertically into 
the yolk, and surrounded by the usual cloud of migrating cells. In front of the 
telopore are two small depressions, /.a., that may represent invaginated appendages. 
Projecting forwards over the telopore is a broad conical process. It probably 
represents a tail lobe such as is seen in Pl. VI, Figs. 48 and 49. 
Back of this lobe is a broad convex area, composed of slightly thickened 
ectoderm and mesoderm, and evidently formed by the concrescence of the pos- 
terior margins of the mesodermic area. The rest of the mesodermic area is 
roughly A-shaped, the two backwardly directed lobes showing notches produced 
by partial concrescence. 
Fic. 56, X 30, not sectioned. This embryo is in about stage C. 
The remnants of the right and left sides have undergone complete median 
fusion. There is now nothing left but an anterior, oesophagus-like tube, oe., 
opening beneath a backwardly directed fold, and two median appendages. The 
posterior one is the larger, and projects backwards a short distance over the floor 
of the depression in which both lie. This depression is roughly boat-shaped, but 
wider and considerably deeper at the posterior end. 
The nervous system is entirely absent, or, at any rate, the ectoderm over the 
bottom of the furrow is not thicker than, or in any way distinguishable from, that 
covering other parts of the embryo. The mesoderm is much thickened under 
what may be considered the body of the embryo, as shown by the dark rim in 
the figure. Buta thin layer of mesoderm extends much farther than this, and 
along its periphery may be seen, in surface views, irregular star-shaped masses of 
cells, deeply imbedded in the yolk, but probably derived from the disintegrated, 
thickened margin of the mesodermic area. 
As seen in sections, the tissues appear to be perfectly normal and healthy. 
There is an indication of a transverse constriction between the two appendages. 
As similar constrictions occur most frequently between the third and fourth 
