Development of the Spiders. 207 



vesicle, the entire evolution agrees absolutely with the deve- 

 lopment of the other Arthropoda, and consists essentially in 

 the following phenomena : — 1, formation of a continuous 

 mesodermic band, and appearance of the sternal fissure di- 

 viding this band into germinative bands ; 2, appearance of 

 the nervous portion within the latter ; 3, growth of the ger- 

 minative bands towards the tergal region, then towards the 

 sternal ; 4, recurvation of the tail, and reversal &c. 



Fig. 5 represents the boundaries of the vitelline vesicle. It 

 commences immediately below the point of union of the two 

 nervous bands — that is to say, a little behind the mouth in the 

 Limuloid stage ; but it nevertheless already presents a sen- 

 sible flattening over all the rest of the thoracic region, and 

 only becomes greatly inflated beneath the abdominal region, 

 where it presents a thick projection : fig. 6 will assist in ren- 

 dering this arrangement intelligible. 



Passage to the young spider. — If we imagine exclusion to 

 take place at this period, the thorax will be completed by the 

 completion of the invagination of the oesophagus and the con- 

 comitant phenomena : the abdomen will close up by the growth 

 of the sternal bands ; and the sternal vesicle, which they 

 compress on all sides, will be absorbed by degrees, and enter 

 gradually within the embryo in proportion as the sternal 

 plates grow over it so as to complete the zonites of the 

 abdomen. There will thus be formed an organism analogous 

 to Hemiaspis limulotdes, while the vitelline vesicle will follow 

 the ordinary course of absorption. 



In the Spiders this is not the case. The new phenomena 

 which rapidly intervene confuse this normal course ; the 

 sternal vesicle does not disappear by absorption, but it is 

 covered up by the exaggerated development of the tergal 

 plate, and is entirely enclosed within the embryo. For 

 this purpose we see all the tergal plate, but especially the four 

 large anterior segments, increase both in length and breadth ; 

 the tergal plate, at first confined to the dorsal surface, thus 

 gradually encroaches upon the vitelline vesicle, which it 

 finally surrounds completely, leaving free only a small oval 

 space indicated in fig. 2 by the line bv, which will be covered 

 up by the development of the sternal plates. At the period 

 when this envelopment is effected, the invagination of the 

 oesophagus and the constriction of the nervous bands have 

 been completed, and the thorax is finally constituted ; further, 

 at the boundary of each of the first four zonites of the abdo- 

 men there is formed a double fold of the mesoderm, com- 

 posed of flat cells, which advances into the mass of the 

 nutritive vitellus so as to constitute a true diaphragmatic 



