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 



ab- 



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

 EMBRYOS ARE REPRESENTED AS IF SEEN EXTENDED ON A PLANE. 



(After Metschnikoff.) 



ch. chelicerae ; pd. pedipalpi ; f l /> 4 . ambulatory appendages ; fe. pecten ; sf. 

 stigmata ; al>. post abdomen (tail). 



behind the pedipalpus), and the abdomen (ab) 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 chelicerae 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 



282 



