DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 49 



witsch, and Locy give figures of this stage in Agalena, in which the 

 cheliceral segment is still united to the cephalic lobe, or is in the act 

 of separating from it. We are unfortunately not able, from this 

 description, to decide the order in which the thoracic segments 

 become differentiated, but it seems as if the most posterior segment 

 (the fourth) arose after the others. The abdominal segments 

 separate from the caudal lobe in the usual order, i.e., from before 

 backward. 



As the germ-band segments, it extends further over the egg ; 

 and not only do its anterior and posterior ends grow towards the 

 dorsal side, but it extends laterally, and may thus, in a few forms 

 {e.g. Pholcus), cover the greater part of the surface of the egg (Fig. 

 25 .4). Seen from the ventral surface, the germ-band now appears 

 broken up into segments, which extend transversely across the whole 

 surface of the egg (Fig. 25 C). The segments appear somewhat 

 narrow and as if separated by broad transverse furrows. The egg 

 therefore somewhat resembles the dorsal surface of a rolled -up 

 Isopod. This condition, however, is not long retained, a lateral 

 -contraction of the germ-band taking place which causes it to draw 

 back again on to the ventral surface (Fig. 25 B), and to lie there in 

 the form of a segmented band. The cephalic and caudal lobes 

 retain their positions unchanged during this process, and, owing to 

 the dorsal extension of the anterior and posterior extremities of the 

 germ-band, they appear closely approximated (Fig. 25 B). In those 

 forms in which the germ-band does, not extend so far over the egg in 

 ■early stages (e.g. Agalena), the cephalic and caudal ends only 

 approach one another on the dorsal surface at a later period. 



The shape of the germ-band becomes modified, the cephalic 

 portion widening and assuming a bilateral, bilobed form ; the abdom- 

 inal segments, further, become separated from the caudal lobe, which 

 has also widened. There may be as many as twelve abdominal 

 segments besides the telson (e.g., Pholcus, Schimkewitsch). The 

 abdomen is thus richly segmented in the Araneid embryo, in direct 

 opposition to its condition in the adult. The complete segmentation 

 of the abdomen does not take place till the later stages, other im- 

 portant modifications in the germ-band preceding it. The first of 

 these to be noted is the appearance of a longitudinal furrow in the 

 ventral middle line (Fig. 28 .4), which is caused by the division of 

 the mesoderm lvhm on the ventral surface into two bands, these 

 subsequently shifting to a more lateral position. The germ-band 

 is in this way divided into two symmetrical halves (Figs. 28 A and 



E 



