TR AC HE AT A. 



359 



p 



cephalic region is distinctly bilobecl, and in the median groove extend- 

 ing along it the stomodjEum has become formed (fig. 19G A). The 

 chelicerse (ch) appear as small 

 rudiments on the first post-oral 

 segment, and the nerve cords are 

 distinctly differentiated and gan- 

 glionated. In the embryonic 

 state there- is one ganglion for 

 each segment. The ganglion in 

 the first segment (that bearing 

 the chelicera?; is very small, but 

 is undoubtedly post-oral. 



At this stage, by a groAvth in 

 which all the three germinal layers 

 have a share, the yolk is com- 

 pletely closed in by the blasto- 

 derm. It is a remarkable fact 

 with only few parallels, and those 

 amongst the Arthropoda, that the 

 blastopore, or point where the 

 embryonic membranes meet in 

 closing in the yolk, is situated on 

 the dorsal surface of the embryo. 



ah 



The general relations of the 



FlG. 195. A FAIRLY-ADVANCED EMBRYO 

 OF THE SCORPION ENVELOPED IN ITS MEM- 

 BRANES. (After Metschnikoff.) 



ch. ckelicers; pd. pedipalpi; p 1 p*. 

 ambulatory appendages; ab. post-abdomen 

 (tail). 



embryo at about this stage are 

 shewn in fig. 195, where the embryo enclosed in its double cellular 

 membrane is seen in a side view. This embryo is about the same 

 age as that seen from the ventral surface in fig. 196 A. 



The general nature of the further changes may easily be gathered 

 from an inspection of fig. 196 B and C, but a few points maybe noted. 



An upper lip or labrum is formed as an unpaired organ in the 

 line between the procephalic lobes. The pedipalpi become chelate 

 before becoming jointed, and the chelicene also early acquire their 

 characteristic form. Rudimentary appendages appear on the six seg- 

 ments behind the ambulatory legs, five of which are distinctly shewn 

 in fig. 195; they persist only on the second segment, where they 

 appear to form the comb-like organs or pectines. The last abdominal 

 segment, i.e. that next the tail, is without provisional appendages. The 

 embryonic tail is divided into six segments including the telson (fig. 

 196 C, ab). The lungs (st) are formed by paired invaginations, the 

 walls of which subsequently become plicated, on the four last segments 

 which bear rudimentary limbs, and simultaneously with the disap- 

 pearance of the rudimentary limbs. 



PseudoSCOrpionidae. The development of Chelifer has been investi- 

 gated by Metschnikoff (436), and although (except that it is provided with 

 trachete instead of pulmonary sacks) it might be supposed to be closely 

 related to Scorpio, yet in its development is strikingly different. 



