320 Thos. H. Montgomery. 



are to be seen in Figs. 7 Go, 80a, 80b, Cliel. G., and lie behind the 

 mouth. The pair of the first abdominal segment are transversely 

 wider than those of the other abdominal segments (Figs. 73, 74, PI. 

 VI; 78, PL VII). 



9. TJie Stage of Reversion. 



Details of this stage are illustrated on PI. VIII, and the upper row 

 of figures (81-86) represent the external conditions that may be 

 described first. 



On comparing Figs. 81-86 of this plate with Figs. 78, 79, PL VII, 

 and Figs. 71-74, PL VI, it will be seen that reversion consists to great 

 extent in a movement of the caudal lobe to a ventral position almost 

 in line with the fourth pair of legs, together with a shortening of the 

 abdomen. An earlier stage of the process is exhibited in Fig. 81, and 

 a later in Fig. 82, while the amount of the movement may be appre- 

 ciated by comparing Fig. 84 wdth Fig. 73. Several other changes 

 are concomitant, to wit: (1) All the abdominal appendages come to 

 lie in approximately the same transverse line with the caudal lobe 

 (Fig. 82) ; (2) the ventral sulcus (Sul v.. Figs. 81, 82, 84) is much 

 shortened and widened so as to be roughly triangular in outline with 

 the base of the triangle resting against the abdomen; (3) the bases 

 of the thoracal limbs are pushed much further dorsad; (4) the extra- 

 embryonic region has become obliterated save in the dorso-median line 

 {H., Figs. 84-86). 



The mechanical causes of reversion will be discussed under the 

 heading, "Summary of Observations." 



Previous to reversion the chelicera lay postoral, but now they are 

 anterior even to the rostrum (Figs. 82, 83), as are their ganglia 

 (Cliel. G., Figs. 83, 85). The relations of these ganglia to the 

 stomodaeum is shown on longitudinal sections in Figs. 89, 90, 91a, 

 and on transverse sections in Figs. 87a, b ; they embrace the 

 stomodaeal tube laterally, are continued anteriorly (dorsally) to it, 

 and are sinking below the ectoblast. 



Other notable changes have been efi^eeted in the cephalic region. 

 The mouth opening is reduced to a slit (^^o.^Fig. 82) ; the stomodaeum 

 has grown inwards still deeper, it is shown in its full extent in Fig. 

 90, and its blind inner end is somewhat dilated. The rostrum is to be 



