682 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 



the translation dorsalwards of the limbs. He compares the 

 dorsal region of the embryo to the arc of a circle, the two ends 

 of which are united by a cord formed by the line of insertion of 

 the limbs. He points out that if you bring the middle of the 

 cord, so stretched between the two ends of the arc, nearer to the 

 summit of the arc, you necessarily cause the two ends of the 

 arc to approach each other, or, in other words, if the insertion 

 of the limbs is drawn up dorsally, the head and tail must ap- 

 proach each other ventrally. 



Barrois takes quite a different view to that of Claparede, 

 which will perhaps be best understood if I quote a translation 

 of his own words. He says : " At the period of the last stage 

 of the embryonic band (the stage represented in PL 31, fig. 7, in 

 the present paper) this latter completely encircles the egg, and 

 its posterior extremity nearly approaches the cephalic region. 

 Finally, the germinal bands, where they unite at the anal lobe 

 (placed above on the dorsal surface), form between them a very 

 acute angle. During the following stages one observes the anal 

 segment separate further and further from the cephalic region, 

 and approach nearer and nearer to the ventral region. This 

 displacement of the anal segment determines, in its turn, a 

 modification in the divergence of the anal bands ; the angle 

 which they form at their junction tends to become more obtuse. 

 The same processes continue regularly till the anal segment 

 comes to occupy the opposite extremity to the cephalic region, 

 a period at which the two germinal bands are placed in the 

 same plane and the two sides of the obtuse angle . end by 

 meeting in a straight line. If we suppose a continuation of the 

 same phenomenon it is clear that the anal segment will come to 

 occupy a position on the ventral surface, and the germinal bands 

 to approach, but in the inverse way, so as to form an angle 

 opposite to that which they formed at first. This condition 

 ends the process by which the posterior extremity of the em- 

 bryonic band, at first directed towards the dorsal side, comes to 

 bend in towards the ventral region." 



Neither of the above explanations is to my mind perfectly 

 satisfactory. The whole phenomenon appears to me to be very 

 simple, and to be caused by the elongation of the dorsal region, 

 i.e. the region on the dorsal surface between the anal and pro- 



