THE NERVOUS SYSTEM, 61 



taking place from behind forward (Locy). Thus while that part 

 of the chain lying in the cephalo-thorax increases greatly in size, 

 that in the abdomen shortens (Fig. 33 A and B). The separate 

 ganglia have already come together to form two thick strands. 

 At first the large ganglionic mass in the cephalo-thorax is still 

 connected with a strand of abdominal ganglia (Fig. 33 A), but, as 

 the growth of the embryo progresses, the latter also is drawn into 

 the common ganglionic mass in the cephalo-thorax, at least this 

 appears to be the way in which we must interpret the process. 

 At first the individual ganglia, though closely pressed together, are 

 still externally distinct (Fig. 33 A), but they become less distinguish- 

 able later (Locy, Morin). 



By the approximation of the two halves of the germ-band, which 

 occurs through the reduction of the yolk-sac, the two portions of 

 the ganglionic chain again approach each other, and transverse 

 commissures now form. 



Balfour, in describing the formation of the transverse commissures, speaks 

 of the appearance of delicate connecting fibres in the dorsal region of the 

 ganglia, and expressly denies that a median ectodermal invagination takes part 

 in the formation of the ganglionic chain. Schimkewitsch, on the contrary, 

 expresses himself quite as decidedly in favour of the presence of such a median 

 strand appearing like a shallow groove between the two approximated ventral 

 nerve-trunks (Fig 4S B and C, p. 92). 



It has already been pointed out (p. 12) that an unpaired median strand 

 participates in the formation of the ganglionic chain in the Scorpiones, and we 

 must assume that this, in both cases, gives rise to the transverse commissures. 



As the concentration of the ganglionic chain commences, the 

 formation of nerve-fibres becomes evident, appearing first in the 

 dorsal part of the ganglia, i.e., at the part where, according to Balfour, 

 the transverse commissures arise (Fig. 33 A and B). When the 

 chain has become highly concentrated, the separate pairs of ganglia 

 are still indicated by the transverse commissures. The elongate 

 ganglionic chain of the earlier stages is now reduced to the com- 

 paratively short and bulky ganglionic mass in the cephalo-thorax, 

 such as is found, though smaller, in the adult. Only one important 

 modification has still to be mentioned. The pair of ganglia 

 belonging to the cheliceral segment, which at first is post-oral, 

 shifts later to a position in front of the mouth, fusing with the 

 brain, and at the same time forming the anterior part of the oeso- 

 phageal commissure. The posterior portion of the latter, i.e., the 

 part lying directly behind the oesophagus, is formed by the ganglia 

 of the pedipalps. 



