DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 



47 



then indicate the longitudinal axis of the embryo. The whole of 

 the triangle thus represents the germ-band or the so-called ventral 

 plate. 



Embryonic envelopes, such as are found in the Scorpiones (p. 4), are wanting 

 in the Araneae. The amniotic fold described by Bruce on the head of the 

 Araneid embryo must without doubt be referred to the infolding which takes 

 place during the formation of 

 the brain. The formation of 

 cuticular larval integuments 

 will be again referred to later 

 (p. 58). 



At about the time when the 

 rudiment of the germ-band 

 (the so-called ventral plate) 

 first appears (Fig. 23 A-C), 

 the egg is said to be flattened 

 on this side, but at a slightly 

 later stage the ventral surface 

 of the embryo appears much 

 arched (Figs. 24 and 25 A), 

 either because this surface has 

 become secondarily convex, or 

 because this special region has 

 not been affected. In Pholcus 

 it appears to be the dorsal part 

 that is flattened (Fig. 25 A and 

 B), and Claparede mentions 

 that in this way the anterior 

 and posterior ends are approxi- 

 mated. 



Fig. 24. — Young embryo of Clubiona incompta show- 

 ing the commencement of segmentation of the 

 germ-band (after Salensky). hi, cephalic lobe; 

 si, caudal lobe ; between these are a few segments 

 in the act of forming. The larger cells outside 

 the region of the germ-band are said to repre- 

 sent blastoderm -cells which are here less crowded 

 (Salensky). 



The segmentation of the germ-band begins with the appearance of 

 a few transverse furrows which mark off a large anterior and a 

 posterior region, as well as several intermediate segments (Fig. 24). 

 These segments at first appear very indistinct, the parts of the body 

 to which they correspond being doubtful. In the youngest segmented 

 stage, three segments were found besides the large anterior and 

 posterior regions (Fig. 24, Salensky, Balfour, Locy, Lexdl). 

 These seem to correspond to the first three thoracic segments. 



According to Locy, we must, however, assume that the three middle segments 

 represent the second, third, and fourth thoracic segments. He believes that the 

 segments develop in the following order ; fourth, third, second, first thoracic 

 segments, then that bearing the pedipalps, and last of all that carrying the 

 chelicerae. The differentiation of the segments would thus take place from 

 behind forward, an exact reversal of the order usually met with in segmented 

 animals. There is a general resemblance between this view and that adopted by 

 Metschnikoff for the Scorpiones, according to which the embryos at first 

 break up into three regions, the anterior corresponding to the cephalic region, 



