pea 
tl 
1886. ] Embryology. 667 
the preceding stage. Sections show that the cavities of the 
limbs are prolongations of the cavities of the corresponding meso- 
dermic somites. The nervous system at first consists of two rows 
of ganglia, one to each somite; these are widely separated in the 
middle line, except in the head and tail lobes, where those of 
Opposite sides are fused. The stomodzum arises as an invagi- 
nation between the ganglionic thickenings of the cheliceral 
oe and immediately below the ventral margin of the cephalic. 
ate 
About the time that the ventral flexure appears, or when the 
. embryo becomes folded upon itself, the proctodzum, heart, lungs, 
trachea, spinning glands and muscles develop. The chelicere 
and pedipalpi appear as postoral structures, but in the course of 
further development they appear as preoral appendages. At an 
early stage the proctodzum is enlarged by the outgrowth of its 
dorsal wall ; from this diverticulum the so-called stercoral pocket 
of the adult is formed. The lateral nerve cords are finally ap- 
Proximated in the middle line, and the posterior or abdominal 
Portion of the nervous system degenerates. The poison glands 
appear as groups of enlarged cells at the bases of the chelicere. 
The spinning glands develop from the ectoderm in the 
region on the ventral side of the proctodzum. e lungs arise 
as -infoldings from a large oval pair of masses of cells, the 
nuclei of which are arranged in parallel lines. From these cells 
the lamalla of the lungs are formed. The heart remains open 
low for a time, communicating freely with the yolk. The 
aorta, at a later period, is constricted off from the mesenteron. 
At least two pairs of the provisional appendages on the abdomen 
Are modified into the spinning mammillz. The remnant of the 
upwardly flexed tail persists for some time as a postanal knob; 
its tip represents the morphological end of the body. 
€ eyes are developed as invaginations of the ectoderm (hypo- 
dermis). The retinal involution becomes constricted off from the 
pe csenmn entirely, and then lies just below that portion of the 
ypodermis which afterwards becomes the vitreous body. In a 
concave depression, on the surface of the latter, the lens arises as 
à lenticular thickening of the cuticula, The mode in which the 
i traverses the eye is essentially similar to the method in 
raich the light reaches the percipient elements of the retina in 
© Vertebrate eye. 
He 
