80 BULLETIN OF THE 



embryo is not the proctodseum, 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 proctodajum. 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 proctodeum is formed as an invagination just ventral to its tip. 

 At this early period it has the appearance shown in the sagittal section, 

 n. VIII. fig. 54. The tail-lobe is now a short thick prominence, and 

 the dorsal fold has nearly disappeared. 



At an early period the proctodaeum 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 (br. stc. 

 PI. VIII. 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 sul> 

 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 (PI. XII. fig. 77), but during reversion they pass through 

 the stages of shortening represented in Pis. 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 chelicerse 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. 



