NOTES ON THE DEVELOPMENT OF THE ARANEINA. 99 



The dorsal region has, however, not only increased in length, 

 but also in definiteness, and a series of transverse markings 

 (fig. 8 a and ^), which are very conspicuous in the case of 

 the four anterior abdominal segments (the segments with 

 rudimentary appendages), have appeared, indicating the 

 limits of segments dorsally. The terga of the somites may, 

 in fact, be said to have become formed. 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 b), 

 have a ventral curvature, or, perhaps more correctly, are 

 formed of two parts, which meet at a large angle open to- 

 wards the ventral surface. The posterior of these parts bears" 

 the four still very conspicuous provisional appendages, and 

 the anterior the six pairs of thoracic appendages. 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 in- 

 terval. 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 (PI. X, fig. 18). 

 The pedipalpi carry a blade on their basal joint. The che- 

 licerEe no longer appear to spring from an independent post- 

 oral segment. 



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

 prominent than before. Sections at this stage show that 

 the internal changes have been nearly as considerable as the 

 external. 



The dorsal region is now formed of a (1) 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 recognise 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 

 commenced to be formed at a much earlier period, and some 

 of them are shown in the long section (PI. IX, fig. 15). 

 In the next stage these cells become differentiated into the 

 somatic and splanchnic mesobla^t 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 



