80 BULLETIN OF THE 
embryo is not the proctodzum, but simply a fold in the dorsal wall of 
the embryo. The pocket resulting from this fold is flattened in a plane 
perpendicular to the sagittal plane, and is not a tubular infolding like 
the real proctodeum., This pocket is lined with ectodermic cells, which 
subsequently form a part of the epidermis at the posterior end of the 
dorsum. By the traction exerted along the median ventral line of the 
body during reversion the tail is drawn downwards and greatly short- 
ened, thus obliterating the pocket. When in the progress of its rever- 
sion the embryo has reached about the stage represented in Figs. 10, 
16, the proctodzeum is formed as an invagination just ventral to its tip. 
At this early period it has the appearance shown in the sagittal section, 
Pl. VIII. fig. 54. The tail-lobe is now a short thick prominence, and 
the dorsal fold has nearly disappeared. 
At an early period the proctodseum is enlarged by the outgrowth of 
its dorsal wall into the form of a capacious pocket, which is retained by 
the embryo throughout its development. This diverticulum (67. ste. 
Pl. VIIL. figs. 55, 56) is the so-called stercoral pocket of the adult. The 
walls of the rectum and the stercoral pocket are composed of columnar 
epithelium, and are closely invested by mesodermic elements. 
The nervous system is characterized during this period by the wide 
separation of the nerve bands and a gradual concentration of their sub- 
stance headwards. The distance between the bands is greatly increased 
by the passage of the yolk from the dorsal to the ventral side through 
the aperture left by their separation. At the period of their great- 
est separation they occupy curved lines along the lateral walls of the 
yolk sack, separated from each other by its diameter. During reversion 
also the actual length of the nerve cords is somewhat decreased. At the 
beginning of this period they reach nearly around the egg from the head- 
to the tail-lobe (Pl. XII. fig. 77), but during reversion they pass through 
the stages of shortening represented in Pls. XI. XII. figs. 72, 71, 70, 78. 
Their connection with the tail-lobe is severed, and the nerve cords grad- 
ually move forwards ; with the absorption of the yolk mass this lateral 
separation is diminished until they are in contact along the ventral line. 
After the process of reversion is well advanced certain cells in the 
bases of the chelicerze become conspicuous from their enlarged condition 
and spongy appearance, which serve to distinguish them sharply from 
surrounding cells. They are the rudiments of the poison glands, and 
although I have not been able to trace an external outlet until a later 
period, it is probable that these cells are derived from an infolding of the 
ectoderm at the point where later an outlet is discernible. 
