DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 



53 



27i e.s 



(Fig. 28). The characteristics of this stage are completed by 

 the appearance of a semicircular furrow on each of the cephalic 

 lobes (Fig. 28 B) ; this, as in the Scorpiones, is connected with 

 the formation of the brain and of the eyes. 



Up to this time the embryo shows a marked dorsal curvature, 

 the continuous elongation of the germ-band causing the latter to 

 assume the form of a nearly complete equatorial ring round the 

 egg, and its cephalic and caudal extremities almost to touch one 

 another (Fig. 27). On the dorsal surface at this stage, therefore, 

 only a small part of the yolk is uncovered by the germ-band ; this 

 represents the future dorsal surface of the Spider, which is now 

 about to com- 

 mence to de- ao-^a 

 velop. This, 

 however, alters 

 in the stages that 

 now follow, and 

 that are charac- 

 terised by the 

 flexure of the 

 embryo gradu- 

 ally changing 

 from a dorsal to 

 a ventral one. 



The striking re- 

 versal of flexure 

 or reversion of 

 the embryo is 

 due to a combi- 

 nation of the fol- 

 lowing changes : 

 the growth of 



the dorsal surface (Balfour), the shortening of the germ-bands 

 (Locy), and the transverse widening of the ventral groove between 

 the two germ-bands. 



Until now the dorsal surface has remained practically undeveloped, 

 the cephalic and caudal extremities of the germ-bands being situated 

 in close proximity to one another on the dorsal surface. The limited 

 cell-area between them now commences to grow, extending anteriorly 

 and posteriorly, thus forcing apart these two lobes. At the same 

 time a shortening of the germ-bands takes place (Locy), accompanied 



Fig. 20. — Transverse section through the thoracic region of an 

 embryo of Agalcna labyrinthica in the same stage as in Fig. 31 A 

 (after Balfodr). The section is made through that region where 

 the ventral yolk-sac protrudes most, ao, aorta ; me.s, mesoderm 

 (primitive segment), which extends on each side up to the dorsal 

 middle line ; vn, ventral cord ; yk, yolk with yolk-cells. 



