318 Thos. II. Montgomery. 



The postero-lateral vesicles are exhibited in Fig. 71, PI. VI (P. L. 

 V.) at the earlier stage when they are still separate from the antero- 

 lateral vesicles (.1. L. V.) ; a transverse section of them in this stage 

 is show^n in Fig. 7Gb, PI. VII. Later their anterior margins and 

 the posterior margins of the antero-lateral vesicles grow to meet each 

 other, as shown in Figs. 79 and 80c; Fig. 80a indicates the method 

 of closnre of each postero-lateral vesicle by overgrowth of its margin. 

 The antero-lateral and postero-lateral vesicles are the beginnings of 

 the optic ganglia. 



Another change is the gradual widening of the ventral sulcus {8ul. 

 v., Figs. 71-74, PL VI; 79, PI. VII), illustrated best, perhaps, by 

 comparison of Figs. 74 and 78. This sulcus has become somewhat 

 diamond-shaped, widest at the juncture of thorax and abdomen, nar- 

 rowing cephalad as well as caudad. It extends from the stomodaeum 

 to about the caudal lobe, and marks the region where there is no meso- 

 blast. By its widening it occasions a still greater reduction of the 

 extraembryonic area (that part in Figs. 71-73 of which the nuclei are 

 indicated by stippling) ; and it is the clearest anticipation of the 

 reversion process soon to follow. 



The chelicera {Chel., Fig. 72, PI. VI) and their ganglia {Chel. 0.) 

 are still poststomial; subsequently each chelieeron acquires a slight 

 maxillary process on its mesial border (Fig. 79, PL VII). The 

 maxillary plates of the pedipalps are well marked (Figs. 71, 72, PL 

 VI; 79, PL VII). The other thoracal appendages {L. 1-L.4, Figs. 

 71-74, 78, 79) are becoming longer and more bent, with four or five 

 joints apiece; in the stage of Figs. 78 and 79 those of opposite sides 

 meet ventrally. 



The four pairs of abdominal appendages {Ah. 2b-Ab. bh, Figs. 71, 

 73, 74, PL VI; 78, PL VII) have gTown larger and the two more 

 posterior pairs (prospective spinnerets) are the largest; all are some- 

 what blunt and rectangular in form; the most posterior pair are 

 shown on section in Fig. 77. The caudal lobe {Caud., Figs. 71-73, 

 PL VI; 78, PL VII) is short, apically rounded and slightly project- 

 ing above the extraembryonic area (Fig. 80e). While the seventh 

 and eighth abdominal segments are at first still distinguishable {Ah. 

 7, Ah. 8, Fig. 71, PL VI), they later fuse with the caudal lobe (Fig. 

 78, PL VII). 



