TRACHEATA. 



433 



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 hinder- 

 most segments smaller provisional appendages which subse- 

 quently disappear, with the possible exception of the second. 

 The foremost segment, immediately behind the procephalic 

 lobes, is very small, and still without a rudiment of the cheli- 

 cerae, which are subsequently formed on it. It would appear 

 from Metschnikoff's figures to 

 be developed later than the 

 other post-oral segments pre- 

 sent at this stage. The still 

 unsegmented tail has become 

 very prominent and makes an 

 angle of 180 with the re- 

 mainder of the body, over the 

 ventral surface of which it is 

 flexed. 



By the time that twelve 

 segments are definitely form- 

 ed, the procephalic region is 

 distinctly bilobed, and in the 

 median groove extending 

 along it the stomodaeum has 

 become formed (fig. 196 A). 

 The chelicerae (ck) appear as 

 small rudiments on the first 

 post-oral segment, and the 



FlG. 195. A FAIRLY-ADVANCED EM- 

 BRYO OF THE SCORPION ENVELOPED IN 



ITS MEMBRANES. (After Metschnikoff. ) 



ch. chelicerae ; pd. pedipalpi ; p^p 4 . 

 ambulatory appendages ; al>. post-abdomen 

 (tail). 



Metschnikoff. 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. 



B. II. 28 





