676 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 



The ventral thickening of the embryo now occupies rather more 

 than half the circumference of the ovum. 



Transverse sections shew that considerable changes have 

 been effected in the constitution of the blastoderm. In the 

 previous stage, the ventral plate was formed of an uniform ex- 

 ternal layer of epiblast, and a continuous internal layer of meso- 

 blast. The mesoblast has now become divided along the whole 

 length of the embryo, except, perhaps, the procephalic lobes, 

 into two lateral bands which are not continuous across the 

 middle line (PL 31, fig. 13 me). It has, moreover, become 

 a much more definite layer, closely attached to the epiblast. 

 Between each mesoblastic band and the adjoining yolk there are 

 placed a few scattered cells, which in a somewhat later stage 

 become the splanchnic mesoblast. These cells are derived from 

 the yolk-cells ; and almost every section contains examples of 

 such cells in the act of joining the mesoblast. 



The epiblast of the ventral plate has not, to any great extent, 

 altered in constitution. It is, perhaps, a shade thinner in the 

 median line than it is laterally. The division of the mesoblast 

 plate into two bands, together, perhaps, with the slight reduc- 

 tion of the epiblast in the median ventral line, gives rise at this 

 stage to an imperfectly marked median groove. 



The dorsal epiblast is still formed of a single layer of flat 

 cells. In the neighbourhood of this layer the yolk nuclei are 

 especially concentrated. The yolk itself remains as before. 



The segments continue to increase regularly, each fresh seg- 

 ment being added in the usual way between the last formed 

 segment and the unsegmented caudal lobe. At the stage when 

 about nine or ten segments have become established, the first 

 rudiments of appendages become visible. At this period (PL 

 30, fig. 4) there is a distinct median ventral groove, extending 

 through the whole length of the embryo, which becomes, how- 

 ever, considerably shallower behind. The procephalic region is 

 distinctly bilobed. The first segment (that of the cheliceras) is 

 better marked off from it than in the previous stage, but is with- 

 out a trace of an appendage, and exhibits therefore, in respect 

 to the development of its appendages, the same retardation that 

 characterised its first appearance. The next five segments, viz. 

 those of the pedipalpi and four ambulatory appendages, present 



