THE DEVELOPMENT OF THE ORGANS. 



59 



3. The Development of the Organs. 

 A. The Nervous System. 

 The ventral chain of ganglia arises shortly after the appearance 

 of the limbs, as ectodermal thickenings on the inner side of their 

 bases (Fig. 32 A). Even before the formation of the limbs, a 

 thickened longitudinal band of ectoderm appeared on each side near 

 the middle line, the two bands being separated by a thin median 

 ectodermal strand. This thickening first appears in front, and grows 

 backwards. A pair of such swellings (ganglia) belongs to each segment. 

 (Balfour, Locy). In the cephalo-thorax there is a pair each for the 

 segments of the chelicerae and the pedipalps, as well as for the four 



■Ui- 



Fig. 32. — Transverse sections through embryos of Theridium maculatum, at different ages (aft.>r 

 Morin). bl, blood-corpuscles ; d, yolk ; <!:, yolk-cells ; ex, limbs ; I, luug-invaginations ; h, 

 rudiment of the ganglionic chain ; us, primitive segments. 



pairs of ambulatory limbs. In the abdomen as many as ten pairs of 

 ganglia (in Pholcus, according to Schimkewitsch, twelve) may appear. 

 The longitudinal commissures connecting the ganglia are represented 

 by thinner portions of ectoderm ; but the transverse connectives are 

 not yet developed, the ganglia being still separated by a thin layer of 

 ectoderm (Fig. 32 A). This is still more striking later, when the 

 two halves of the germ-band shift apart (Fig. 32 B, n). The two 

 strands of the ventral chain of ganglia, which at first lay near the 

 middle line, are widest apart when the yolk is pressed towards the 

 ventral side, so as to form the so-called yolk-sac (Figs. 32 B and 29, 



