The Development of Theridium. 333 



ambulatory appendages, or (2) the pedipalps and three anterior 

 ambulatory appendages ; the latter alternative is probably the correct 

 one, for the stage with five segments (pedipalpal and four ambula- 

 tory) shows the most posterior more distinct than the most anterior. 

 The cheliceral segment does not abstrict from the posterior margin 

 of the head lobe until the other segments exhibit beginnings of appen- 

 dages. The appendages are produced by lateral movement of the 

 mesoblast away from the midline, this occasioning also the clear 

 median line or ventral sulcus (region from which the mesoblast has 

 withdrawn) ; with this division of the mesoblast is formed a pair of 

 coelomic sacs for each of the segments mentioned. The stomodaeum 

 arises as an ectoblastic invagination of the head lobe anterior to its 

 middle point; its external lips are mere ectoblastic thickenings, dis- 

 tinct at first from the rostrum ; it is at first anterior to the chelicera 

 and their ganglia, but the latter move anterior to it at the time of 

 reversion. The head lobe contains two pairs of coelomic sacs distinct 

 from the start: the more posterior and smaller cheliceral sacs, and 

 the more anterior and much more voluminous rostral sacs. The head 

 region possesses, accordingly, two segments, demarcable not externally, 

 but by mesoblast sacs: the more anterior and larger of these seg- 

 ments is the rostral, and as its appendages are to be considered the 

 two small rostral tubercles, which arise later than the chelicera as 

 a pair of small tubercles just anterior to the stomodaeum, and which 

 now fuse to make the prestomial rostrum ; the rostral coelomic sacs 

 extend into their bases. The rostrum, accordingly, represents a paii- 

 of true preoral appendages, of which the rostral sacs constitute the 

 coelom and the cerebral ganglia the neuromeres. There is no evi- 

 dence of other preoral appendages. The cephalothorax consists of 

 seven segments. The mesoblast sacs of the cephalothorax are at first 

 latero-ventral in position and (except those of the rostrum) with 

 coelomic spaces only in the region of the appendages ; gradually they 

 extend dorsad, but it is mainly the reversion of the embryo that 

 brings those of opposite sides in proximity on the dorsal aspect of 

 the embryo. Of the appendages both chelicera and pedipalps de- 

 velop maxillary processes, but those of only the pedipalps persist into 

 the adult. 



