102 PATTEN. [Vou. XII. 
EXPLANATION OF PLATE II. 
Nearly all the figures were outlined with a camera and drawn to the same scale. They were 
made from mounted and cleared specimens, viewed with a raised condenser and wide open 
diaphragm. In this way they appear bright red on a clear yellow field ; the elevations showing dark, 
the depressions light. 
The embryos 4 to Z represent normal stages introduced here for comparison with the abnormal 
ones. Figs. 1 and 2 belong to an earlier series, where a different method of designating the stages 
has been adopted. 
Fic. 1, X 50. Surface view of a normal embryo, stained and cleared in oil of 
cedar. The cephalic lobes form a clearly outlined, semicircular plate of ectoderm, 
thickened on each side. There is no trace of an oesophagus, but very faint indi- 
cations of the cheliceral segment may be seen on the posterior margin of the lobes. 
The next three thoracic metameres are fully formed. In reflected light they 
appear, in surface views of opaque preparations, as gentle undulations of the 
surface, forming a series of ridges and valleys. The lateral ends of the low 
ridges are bent backwards and unite with each other near the margin of the 
mesodermic area. When the eggs are cleared in oil, the mesodermic segments 
are seen beneath, and about coextensive with the surface ridges. The fifth seg- 
ment is just separating off from the anal plate. Along the median line of the 
latter is a slit-like invagination to form the Ze/opfore, from which a sheet of inner 
layer cells extends laterally and forwards, while beneath the invagination a line of 
cells extends into the yolk. 
Fic. 2, X 50. Surface view of an embryo about 10 days old. The drawing was 
made from studies of the opaque embryo in alcohol, and from studies of the same 
embryo cleared in clove oil and stained in borax carmine. The sixth segment has 
appeared on the anterior margin of the anal plate. The lateral ends of the four 
preceding mesodermic somites have become confluent, and the mesodermic area 
thus formed is provided with a distinct thickening along its lateral margins, m. a., 
forming what I have called the “ thickened rim of the mesodermic area.” 
The cheliceral segment is now quite distinct, and a faint, dark band is seen in 
front of the cheliceral segment connecting the two sides of the cephalic lobes. 
Fic. 3, X 55. Surface view of embryo about twelve days old, stained in borax 
carmine and cleared in oil of cedar. 
All six thoracic segments are now formed. The sixth, as was the case at a 
corresponding age with each of the preceding segments except the chelicerae, is 
very plainly marked by its dark color, and by the sharp furrows that lie on either 
side of it. , 
The second, third, and fourth pairs of appendages have appeared, and on either 
side of them are faint spots, that are perhaps the beginnings of the sense-organs, 
seen in this region more clearly at a later stage. 
Fic. 4, X 50. Surface view of a normal embryo 14 days old, stained in borax 
carmine and cleared in oil of cedar. The three pairs of appendages are more 
conspicuously developed, the oesophagus has appeared at the anterior margin of 
the cephalic lobes, and the brain and ventral nerve-cords are now easily distin- 
guished by the characteristic mottlings, due to the presence of many minute pits, 
precisely similar to those described and figured by me in scorpions. The appear- 
ance is due to the presence in the nerve-cord and cephalic lobes of numerous 
independent, bud-like thickenings of the ectoderm, which in histological special+ 
