684 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 



The posterior terga (fig. 8 a) are very narrow compared to the 

 anterior. 



The caudal protuberance is more prominent than it was, and 

 somewhat bilobed ; it is continued on each side into one of the 

 bands, into which the ventral plate is divided. These bands, as 

 is best seen in side view (fig. 8 &), have a ventral curvature, or, 

 perhaps more correctly, are formed of two parts, which meet at 

 a large angle open towards the ventral surface. The posterior 

 of these parts bears the four still very conspicuous provisional 

 appendages, and the anterior the six pairs of thoracic append- 

 ages. The four ambulatory appendages are now seven-jointed, 

 as in the adult, but though longer than in the previous stage 

 they do not any longer cross or even meet in the middle line, but 

 are, on the contrary, separated by a very considerable interval. 

 This is due to the great distension by the yolk of the ventral 

 part of the body, in the interval between the two parts of the 

 original ventral plate. The amount of this yolk may be gathered 

 from the section (PL 32, fig. 18). The pedipalpi carry a blade 

 on their basal joint. The chelicerae no longer appear to spring 

 from an independent postoral segment. 



There is a conspicuous lower lip, but the upper is less 

 prominent than before. Sections at this stage shew that the 

 internal changes have been nearly as considerable as the ex- 

 ternal. 



The dorsal region is now formed of a (i) flattened layer of 

 epiblast cells, and a (2) fairly thick layer of large and rather 

 characteristic cells which any one who has studied sections of 

 spider's embryos will recognize as derivatives of the yolk. 

 These cells are not, therefore, derived from prolongations of the 

 somatic and splanchnic layers of the already formed somites, 

 but are new formations derived from the yolk. They com- 

 menced to be formed at a much earlier period, and some of 

 them are shewn in the longitudinal section (PL 31, fig. 15). In 

 the next stage these cells become differentiated into the somatic 

 and splanchnic mesdblast layers of the dorsal region of the 

 embryo. 



In the dorsal region of the abdomen the heart has already 

 become established. So far as I have been able to make out it 

 is formed from a solid cord of the cells of the dorsal region. 



