200 Dr. J. Barrois on the 



( Scheitelplatten of German authors) . These lobes are bounded 

 within and above by a groove bordered on all sides by a pro- 

 jecting crest due to a thickening of the cells of the blastoderm. 

 This crest forms between the lobes a projecting tongue, no 

 doubt the representative of the labrum ( Vord&rkopf of the Ger- 

 mans), and is afterwards produced on each side into two semi- 

 circular thickenings which surround the cephalic lobes on the 

 outside. The base of the tongue, which represents the labrum, 

 presents a deepening of its median furrow, which indicates the 

 commencement of the invagination of the oesophagus and 

 constitutes the mouth. We see that the structure of the 

 cephalic region is much more complex than Claparede sup- 

 posed, and that we can recognize in it all the constituent parts 

 of the head of the Arthropoda ; its aspect presents striking 

 analogies with what Metsclmikoff has described in the Scor- 

 pions : the labium does not appear to be formed until a little 

 later, and independently of the cephalic portion, at the expense 

 of a median thickening of the mesoderm, placed between the 

 first union of the two nervous bands below the oesophagus 

 (see PI. IX. figs. 2,3,/). 



As regards the number of zonites of the embryonic band, I 

 have found it, at least in Epe'ira diadema, greater than that 

 given by Claparede. The number of segments following the 

 six thoracic ones appears to me in general to amount to ten 

 (including the anal hood) ; the first four are more developed 

 and constantly bear rudiments of limbs ; all the others have 

 always seemed to me destitute of appendages. 



Besides the rudiment of a limb, or the vacant space which 

 takes its place and forms the median part directly superim- 

 posed upon the germinative band of each of the segments, all 

 the zonites of the abdomen always present, towards the last 

 periods of the stage of the embryonic band, two lateral por- 

 tions, a sternal and a tergal plate, formed by thickened cells 

 of the blastoderm, beneath which the mesoderm of the germi- 

 native bands has begun to spread out in a thin layer. The 

 tergals are always perfectly visible, and are wider in the four 

 anterior abdominal segments ; the sternals in the posterior 

 segments appear under the form of perfectly distinct, thin, 

 elongated plates, which may easily be mistaken for rudi- 

 ments of limbs (as no doubt has been done by Claparede) if 

 one does not pay particular attention to their position. In 

 reality the six posterior segments have always seemed to me 

 to be destitute of appendages ; their median portion, superim- 

 posed upon the germinative band, being only occupied by a 

 paler line. 



This appearance of the sternal and tergal arcs shows us that 



