NOTES ON THE DEVELOPMENT OF THE ARANETNA. 679 



The epiblast at this stage has also undergone important 

 changes. Along the median ventral groove it has become very 

 thin. On each side of this groove it exhibits in each append- 

 age-bearing somite a well-marked thickening, which gives in 

 surface views the appearance of a slightly raised area (PL 30, 

 fig. 5), between each appendage and the median line. These 

 thickenings are the first rudiments of the ventral nerve gang- 

 lia. The ventral nerve cord at this stage is formed of two 

 ridge-like thickenings of the epiblast, widely separated in the 

 median line, each of which is constituted of a series of raised 

 divisions the ganglia united by shorter, less prominent divi- 

 sions (fig. 14, vg). The nerve cords are formed from before 

 backwards, and are not at this stage found in the hinder seg- 

 ments. There is a distinct ganglionic thickening for the cheliccrce 

 quite independent of the procephalic lobes. 



In the procephalic lobes the epiblast is much thickened, 

 and is formed of several rows of cells. The greater part of 

 it is destined to give rise to the supra-cesophageal ganglia. 



During the various changes which have been described the 

 blastoderm cells have been continually dividing, and, together 

 with their nuclei, have become considerably smaller than at 

 first. The yolk cells have in the meantime remained much as 

 before, and are, therefore, considerably larger than the nuclei 

 of the blastoderm cells. They are more numerous than in the 

 earlier stages, but are still surrounded by a protoplasmic body, 

 which is continued into a protoplasmic reticulum. The yolk is 

 still divided up into polygonal segments, but from sections it 

 would appear that the nuclei are more numerous than the seg- 

 ments, though I have failed to arrive at quite definite conclu- 

 sions on this point. 



As development proceeds the appendages grow longer, and 

 gradually bend inwards. They become very soon divided by 

 a series of ring-like constrictions which constitute the first indi- 

 cations of the future joints (PL 30, fig. 6). The full number of 

 joints are not at once reached, but in the ambulatory ap- 

 pendages five only appear at first to be formed. There are four 

 joints in the pedipalpi, while the chelicerae do not exhibit any 

 signs of becoming jointed till somewhat later. The primitive 

 presence of only five joints in the ambulatory appendages 



