MUSEUM OF COMPARATIVE ZOOLOGY. 77 



cerse has a separate pair of ganglia which ultimately disappear, serving 

 only to aid in the formation cf the circuraoesophageal commissure. At 

 an early stage, then, the nervous elements consist of two rows of ganglia, 

 a pair of ganglia for each somite, which are widely separated except in the 

 head lobe and the tail lobe, where they are continuous in the median 

 line. 



Another important growth on the part of the ectoderm leads to the 

 formation of the stomodseum, which arises as a simple tubular infolding 

 between the ganglionic thickenings of the cheliceral somites, and imme- 

 diately below the ventral margin of the cephalic plate. It becomes ex- 

 panded at its deep end into a sort of pocket, but it has only a small 

 external opening. The walls of the stomodaeum are composed of cells, 

 two or three rows deep, which are elongated and somewhat wedge-shaped 

 rather than distinctly columnar. 



4. The period of reversion is marked by the origin of^ many important 

 organs : proctodaeum, heart, lungs, tracheae, spinning glands, muscles, 

 etc. The embryo undergoes great changes in external form, gradually 

 passing from the condition represented in PI. II. fig. 8, where the ven- 

 tral surface of the embryo is uniformly convex, and occupies an arc of 

 about 300°, to a form (PI. II. fig. 11) in which the ventral surface is 

 folded upon itself. 



As a prelude to reversion the tail-lobe of the embryo becomes promi- 

 nent, being raised from the surface of the egg. The early steps in the 

 process of reversion will be best understood from the examination of a 

 series of dorsal views. Fig. 13 (PI. III.) presents the dorsal aspect at 

 the beginning of reversion, and Fig. 8 (PI. II.) a side view at nearly the 

 same stage. The tail-lobe has lost its broad rounded character, and is 

 being changed into a more distinctively caudate structure. It still re- 

 mains nearly in contact with the cephalic lobe. Tlie dorsal element? 

 ("terga" of Barrois) have begun their upward growth, and appear in 

 the figure as four pairs of prominent lateral elevations. A corres])onding 

 growth of the abdominal segments is also in progress ; the dorsal elements 

 growing upward finally meet in the median line of the back. 



Each of the lobes of the procephalic plate has a semilunar form, and is 

 composed of a central area, apparently separated from a marginal rim by 

 means of a deep fold (PI. IV. fig. 23). The prominent upper lip (/r.) 

 is apparently an outgrowth of the ventral border of the cephalic plate, 

 and overhangs the entrance to the stomodreum (sd.). 



The chelicerte (1 ap]}.) and the pedipalpi (2 «/?/>.) both appear as post- 

 oral structures. The prominent ganglia (gji.) belunging to the cheli- 



