358 



SCORPIONID^E. 



pyriform, the pointed end being the posterior. At this extremity 



there is a special thickening 

 which is perhaps equivalent 

 to the primitive cumulus of 

 Spiders. The germinal disc 

 continues gradually to spread 

 over the yolk, but the ori- 

 ginal pyriform area is thicker 

 than the remainder, and is 

 marked off anteriorly and 

 posteriorly by a shallow 

 furrow. It constitutes a 

 structure corresponding with 

 the ventral plate of other 

 Tracheata. It soon becomes 

 grooved by a shallow longi- 

 tudinal furrow (fig. 194 A) 

 which subsequently becomes 

 less distinct. It is then 

 divided by two transverse 

 lines into three parts 1 . 



In succeeding stages the 

 anterior of the three parts 

 is clearly marked out as the 

 procephalic lobe and soon 

 becomes somewhat broader. 



Fresh segments are added from before backwards, and the whole 

 ventral plate increases rapidly in length (fig. 194 B). 



When ten segments have become formed, appendages appear as 

 paired outgrowths of the nine posterior segments (fig. 194 C). The 

 second segment bears the pedipalpi, the four succeeding segments the 

 four ambulatory appendages, and the four hindermost segments 

 smaller provisional appendages which subsequently disappear, with 

 the possible exception of the second. The foremost segment, imme- 

 diately behind the procephalic lobes, is very small, and still without 

 a rudiment of the chelicera3, which are subsequently formed on it. It 

 would appear from Metschnikoff's figures to be developed later than 

 the other post-oral segments present at this stage. The still unseg- 

 mented tail has become very prominent and makes an angle of 180 

 with the remainder of the body, over the ventral surface of which it 

 is flexed. 



By the time that twelve segments are definitely formed, the pro- 



1 The exact fate of the three original segments is left somewhat obscure by Metsch- 

 uikoff. He believes however that the anterior segment forms the procephalic lobes, the 

 posterior probably the telson and five adjoining caudal segments, and the middle one 

 the remainder of the body. This view does not appear to me quite satisfactory, since on 

 the analogy of Spiders and other Arthropoda the fresh somites ought to be added by 

 a continuous segmentation of the posterior lobe. 



FIG. 194. THREE SURFACE VIEWS OF THE 

 VENTRAL PLATE OF A DEVELOPING SCORPION. 

 (After Metschnikoff.) 



A. Before segmentation. 



B. After five segments have become formed. 



C. After the appendages have begun to be 



formed. 



