MUSEUM OF COMPARATIVE ZOOLOGY. 79 
the embryo becomes more folded upon itself ventrally, as shown in 
Pl. II. fig. 10, and the legs, increasing in length, gradually approach and 
finally overlap each other in the median line. The embryo has now 
acquired a strong ventral flexure — the reversion is completed. 
During this period the bases of the chelicere in growing have moved 
forwards and met in the median plane, so that they appear as pre-oral 
appendages. There has also appeared between their bases a prominent 
outgrowth to form the rostrum. 
Balfour (’80, p. 180) endeavors to account for the process of rever- 
sion as the result of a rapid ‘‘ elongation of the dorsal region, that is, the 
region on the dorsal surface between the anal and the procephalic lobes.” 
I understand by this that it is to the growth of the ectodermic cells of 
the dorsal region that he would ascribe the elongation of the dorsal surface. 
I shall endeavor to show presently that this explanation is not sufficient 
to account for the changes which actually take place during reversion. 
The growth of the derivatives from the ectoderm during the period 
under consideration is very great. At the beginning of the period the 
stomodeum forms a pocket-shaped invagination with a small external 
opening. Its calibre diminishes, except at its anterior end ; it continues 
to grow inwardly, and at length forms an arched tubular organ, with its 
free end directed backward, and projecting some distance into the yolk. 
Near the close of the period its deep end becomes somewhat enlarged to 
form the rudiment of the sucking stomach. To the latter are attached a 
vertical muscle (mz. wrt. Pl. IX. fig. 62) extending to the dorsal wall of 
the embryo, and two lateral muscles (mw. /at.). 
The proctodzeum is a later formation, which makes its appearance as an 
infolding at the tip of the tail-lobe some time after the beginning of this 
period. ‘The relation of the tail-lobe to the rest of the body is best ap- 
preciated from sections, since it is not always evident from surface views 
that there is a deep fold which serves to separate it from the underlying 
portion of the dorsal surface. The prominence which it attains and the 
changes which it undergoes are readily traceable in a series of figures 
from successive stages during reversion (Pl. VIII. figs. 50-54). The 
strong resemblance of the condition shown in Fig. 50 to that which 
Bobretzky (74, Fig. 15) has figured for Oniscus at an apparently similar 
stage of development, misled me into the supposition that I should find 
the proctodeum of Agelena developing in the manner described by him 
for Oniscus. But such is not the case. In Agelena the tip of the lobe 
is the tip of the tail—the morphological end of the body, and the 
' depression which separates this lobe from the neighboring portion of the 
