Development of the Spiders. 205 



menon of inversion resides essentially in the caudal region ; 

 the protrusion of the nutritive vitellus through the sternal 

 fissure is a purely passive feature of development, and deter- 

 mined by the pushing back of the vitellus towards the poste- 

 rior region — a movement resulting from the three processes 

 just indicated, all three of which produce, in different degrees, 

 the effect of restricting the capacity of the embryo. 



Limuloid Stage. — The three series of modifications that we 

 have just indicated are not all produced with the same 

 rapidity ; the last two are more rapid, and are seen com- 

 pletely finished at a period when the union of the two 

 nervous bands has only commenced under the cephalic region. 

 At this period, in consequence of these changes, the embryo 

 presents a very remarkable aspect (see fig. 1). It is divided 

 into two distinct parts, answering to the thoracic and abdo- 

 minal segments, and which seem to me to correspond in a 

 very striking manner to the two divisions of the body in the 

 Xiphosura : the posterior portion, or tergal plate, is formed by 

 the amalgamation of all the tergal arcs ; we can recognize 

 in it each of the segments which formed the abdomen in 

 the embryonic band ; but here this abdomen is divided into 

 two parts — a preabdomen composed of six segments, and a 

 narrow postabdomen formed of four segments. The pre- 

 abdomen is itself subdivided into four broad zonites bearing- 

 appendages already indicated in the embryonic band, and 

 two much narrower ones following the former. The anal 

 segment at first appears simple ; and in Epeira diadema it is 

 impossible to discover in it any trace of division. Never- 

 theless, by examining the sternal arcs we ascertain that 

 the one corresponding to the anal segment is divided into 

 three distinct pieces, which shows that this anal segment 

 is here equivalent to three segments soldered together. This 

 interpretation of the anal segment is not without interest, if 

 we collate it with the fact, observed in Pholcus by Claparede, 

 of the early division of the anal segment into three distinct 

 segments. The fact observed in Epeira diadema shows that 

 this is perhaps general, and that this division, although rarely 

 so early, none the less virtually exists. 



This multiple value of the anal segment brings the number 

 of segments of the entire abdomen to twelve, and that of the 

 postabdomen to six; and this agrees within a segment with 

 the exact number of zonites in the abdomen of the Scorpions, 

 and corresponds exactly for the postabdomen. The stage 

 fig. 1, so like the king-crabs in the division of the body into 

 two portions, would thus prove to come very near the Scor- 

 pions in the number of zonites in the different divisions. 



