THE EMBEYOLOGY OP A SCORPION. 133 



(3) The abdominal appendages appear as thickenings of the 

 epi- and meso-blast on the seventh to twelfth somites (Stage K) . 

 The first pair (genital opercula) does not develop further till 

 a late stage (L). 



The second pair (pectines) form a number of short longi- 

 tudinal ridges on the surface of the abdomen (Stage K). They 

 then separate from the body, the separation beginning at their 

 outer ends (Stage M). 



The third to sixth pairs (gill- books) begin to be pushed in 

 (Stage L). The inpushing becomes deeper, and begins to be 

 divided up (Stage M), and by the time the embryo is hatched 

 they have attained their adult condition in every respect except 

 size and number of lamellae. 



(4) The cerebral ganglion and central eyes begin as a pair of 

 invaginations on the cephalic lobes. These invaginations 

 meet in the middle (Stage K). The cerebral ganglion is formed 

 from the sides of the invaginations, which rapidly become 

 shallower and unite so as to open in the middle line. The 

 dorsal surface of the invagination becomes thickened to form 

 the retina of the central eyes (Stage L). 



The brain becomes entirely separate from the hypodermis, 

 the invagination remaining to form the eyes (Stage M). The 

 invagination closes up and its lumen disappears. The cells of 

 its lower layer form the post-retinal membrane. Those of the 

 upper layer form the retina, and come in contact with the hy- 

 podermis on the top of the head, which is thickened in this 

 region to form the vitreous layer. The retinal cells become 

 deeply pigmented (Stage N). 



(5) The lateral eyes form as cup-shaped thickenings of the 

 hypodermis in the " optic area,^' the cells of which become 

 pigmented. There is no invagination, and they consist of a 

 single layer (Stage Mj. 



(6) The ventral nervous system forms as a pair of thickened 

 segmented bands, one on each side of the neural groove 

 (Stage I). The nerve-cords sink down, a thin layer of hypo- 

 dermis growing over them. There is at this time (Stage K) 

 a distinct postoral pair of ganglia for the chelicerse. The cells 



