204 Dr. J. Barrois on the 



thickening, the still persistent remains of the germinative 

 bands. 



3. Posterior extremity. — At the period of the last stage of 

 the embryonic band this latter runs completely round the egg, 

 and its posterior extremity comes nearly into contact with the 

 cephalic region ; lastly the germinative bands, in running to 

 unite in the anal hood (placed high up on the dorsal surface) , 

 form between them a very acute angle. In the subsequent 

 stages we see the anal segment gradually increase its distance 

 from the cephalic region, and pass more and more towards 

 the ventral region ; this displacement of the anal segment in 

 its turn causes a modification in the separation of the germina- 

 tive bands, the angle formed by their union tending to become 

 more and more obtuse. The same processes continue regu- 

 larly until the anal segment has come to occupy the extremity 

 opposite to the cephalic region, at which period the two ger- 

 minative bands have come into the same plane, the progres- 

 sive separation of the two sides of the obtuse angle finally 

 producing a straight line. If we imagine a continuation of 

 the same phenomena, we shall see the anal segment come to 

 occupy a position upon the ventral surface, and the germina- 

 tive bands approach each other again, but in the inverse 

 sense, so as to form an angle opposed to that which they 

 formed at the commencement ; this state concludes the pro- 

 cess by which the posterior extremity of the embryonic bands, 

 at first directed towards the dorsal side, becomes recurved 

 towards the ventral region. 



If we attempt to trace the same phenomena in Arthropods 

 with a projecting caudal part (e. g. the Phryganidaj) , in which 

 the recurvation of the caudal part is regarded as the cause and 

 origin of the whole inversion of the embryo in the egg, we 

 shall find exactly the same phenomena (1, retractation of the 

 caudal region and separation of the germinative bands ; 2, 

 arrival of the anal segment at the posterior pole of the egg, 

 and arrival of the embryonic bands in the same plane ; 

 3, arrival of the anal segment upon the ventral surface, and 

 approximation of the embryonic bands in the inverse direc- 

 tion). The only difference consists in that the same pheno- 

 mena will take place in the depth instead of at the surface of 

 the egg : and this difference is not at all essential ; for we may 

 still recognize in the Spiders a remnant of a tendency of the 

 caudal portion to detach itself from the rest of the vitellus ; 

 the anal segment almost always forms at first a strong pro- 

 jection, which indicates the commencement of this process. 

 We shall therefore arrive at the conclusion that in the Spiders, 

 as in the other Arthropoda, the cause and origin of the pheno- 



