Development of the Spiders. 203 



it carries them with it towards the posterior part : the vertical 

 partition is thus carried to the end of the thorax, which it 

 completely divides into two symmetrical halves (fig. 6, cce) , at 

 first filled with nutritive vitellus ; the transverse partitions 

 are simply spread out and brought nearer to each other, so as 

 to form a continuous muscular plane, situated at first in the 

 fore part of the thorax. In thus approaching each other these 

 two planes carry with them the two nervous bands, which 

 they always immediately cover ; and thus is effected the ap- 

 proximation of these latter to form a central mass, which 

 directly afterwards divides into ganglia. This union of the 

 nervous bands is therefore itself only a consequence of the 

 growth of the invagination of the oesophagus ; and all the 

 phenomena which give origin to the thorax may be referred 

 to this single cause. 



When once these processes are completely terminated, the 

 thorax is formed in all its more essential features. One of 

 the chief effects of its formation is to restrict the cavity of 

 the whole anterior part of the embryo ; the vitellus is driven 

 back towards the posterior part ; and there only remains in 

 the thorax the portion which will give origin to the caeca of 

 the stomach. 



2. Median portion. — In this portion we see each of the 

 tergal and sternal pieces increase to unite with that of the 

 opposite side, so as to form a complete arch. The tergal arcs 

 grow rapidly ; and we are soon able to distinguish perfectly 

 distinct hemizonites formed by their union in the dorsal 

 region. Of these we always distinguish four much wider than 

 the others, formed by the first four abdominal segments ; with 

 the other narrower ones which follow them they constitute a 

 dorsal plate, to which we shall refer again. The sternal arcs 

 likewise increase, but more slowly ; and, moreover, their union 

 is prevented by the projection of the nutritive vitellus, which, 

 pushed backward by the contraction of the thoracic region and 

 by the formation of the complete tergal arches, protrudes 

 through the sternal fissure, passing between the two hemi- 

 sternals. At the same time that the thickening of the cells of 

 the external lamella, which constitutes the sternal and tergal 

 plates, advances more and more towards the median lines of 

 the back and belly, the thin mesodermic layer subjacent to it 

 likewise increases. At the period when the tergals have 

 united to form complete arches, we also find a complete meso- 

 dermic layer, which extends beneath the whole of the tergal 

 plate, and which from the first begins to thicken upon the 

 median line, to give origin to the dorsal vessel ; at the edges, 

 between the sternals and the lateral region, there is always a 



