60 



ARACHNIDA. 



p. 53). Before this happens, however, various modifications have 

 taken place in the length of the chain ; the ganglia also gradually 

 lose their connection with the ectoderm, and come to lie internal to 

 it (Fig. 48 B and C, p. 92). 



mu 



Fig. 33. — Longitudinal sections through embryos of Theridium mcwulatitm. at different ages 

 (after Morin). a, anus ; U, blood-corpuscles ; d, yolk ; dz, yolk -cells ; g, brain ; k, heart ; I, 

 hepatic lobes ; m, mouth ; md, rudiment of the enteron ; mu, muscles ; n, ventral ganglia ; 

 rb, rectal vesicle ; s, indications of external segmentation ; sp, splanchnic layer of the 

 mesoderm ; vd, stomodaeum. 



When the germ-band still shows a dorsal curvature, the ventral 

 chain of ganglia also extends round the whole of the yolk, but, as 

 the germ-band reverses its flexure, it shortens, the concentration 



