434 Avery E. Lambert. 



The sensory pits arise first in tlie ganglionic areas of the cephalic 

 plate. Later, hoAvever, they appear in the thickened median por- 

 tions, or neuromeres, of the thoracic and abdominal segments, as well 

 as in other parts of the embryo which are destined to enter into 

 the formation of the sense organs. 



The appearance of these primitive sensory pits in the cephalic 

 plate, in the rudiment of the nerve cord, and in other sensory 

 structures of Epeira, strongly favors the view that the central nerv- 

 ous system arises, phylogenetically, not by the multiplication of 

 simple neural elements, but by the transfonnation and aggregation 

 of prhniiive sense organs into the parts from which the nervous 

 system is derived. 



Stage IV. Fig. 23.- — The first change to be noted in this stage 

 is the increase in the size of the appendages which have become 

 longer and thicker in a marked degree, the pedipalps having developed 

 a distinct coxal portion. Slight transverse constrictions appear in the 

 pedipalps and in the thoracic appendages, which are indicative of 

 future segmentation. 



In the body of the embryo the wide separation of the lateral 

 plates has caused the thoracic appendages to be further removed 

 from the median line than is the case with the ap])endages of the 

 head. Small knob-like projections appear on the second, third, fourth, 

 and fifth abdominal segments. These are rudimentaiy abdominal 

 appendages. There has been some discussion concerning the appear- 

 ance of a first, limbless abdominal segment in the Araneina. Schim- 

 kewitsch (38) and Bruce (5) report a segment which fails to bear 

 any trace of an appendage, appearing between the last thoracic 

 segment and the first limb-bearing segment of the abdomen. Kishi- 

 nouye (19) also found that appendages were wanting on the first 

 abdominal segment of the spiders which he studied. 



On the other hand, Balfour (1) and Locy (23) show in their 

 drawings appendages on the first segment of the abdomen. Korschelt 

 and Heider in the '^Lehrbuch der vergleichenden Entwicklungs- 

 geschichte" figure an Arancid embryo which has a pair of append- 

 ages on the first abdominal segment, as well as on the others, making 

 five pairs of abdominal appendages in all. The accuracy of this 



