TRACHEATA. 



435 



(fig. 197 A). There then appear one or more clear segments on the surface 

 of these, and finally a complete layer of cells is formed round the central 

 yolk spheres (fig. 197 B), which latter subsequently agglomerate into a 

 central mass. The superficial cells form what may be called a blastoderm, 

 which soon becomes divided into two layers (fig. 197 C). There now 

 appears a single pair of appendages (the pedipalpi) (fig. 198 A, pd), while at 

 the same time the front end of the embryo grows out into a remarkable 

 proboscis-like prominence a temporary upper lip (concealed in the figure 



FIG. 196. THREE STAGES IN THE DEVELOPMENT OF THE SCORPION. THE 

 EMBRYOS ARE REPRESENTED AS IF SEEN EXTENDED ON A PLANE. 

 (After Metschnikoff.) 



ch. chelicerre ; pd. pedipalpi ; /* p*. ambulatory appendages ; pe. pecten ; st. 

 stigmata ; al>. post abdomen (tail). 



behind the pedipalpus), and the abdomen (ad) becomes bent forwards to- 

 wards the ventral surface. In this very rudimentary condition, after under- 

 going an ecdysis, the larva is hatched, although it still remains attached to 

 its parent. After hatching it grows rapidly, and becomes filled with a 

 peculiar transparent material. The first pair of ambulatory appendages is 

 formed behind the pedipalpi and then the three suceeding pairs, while at the 

 same time the chelicera: appear as small rudiments in front. External signs 

 of segmentation have not yet appeared, but about this period the nervous 

 system is formed. The supra-cesophageal ganglia are especially distinct, 

 and provided with a central cavity, probably formed by an invagination, as 

 in other Arachnida. In the succeeding stages (fig. 198 B) four provisional 



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