ox THE DEVELOPMENT OF LIMULU« LONGISPIXA. '^7^ 



dorsal median lines, i.e., all rlie seg'nients have their irreatest antero- 

 posterior extension on this line, and bex'onie shorter and converge, like 

 tlie frame -pieces of a folding fan, toAvards hoth t'lie ventral and dorsal 

 median lines. Therefore the external ends of the brain are pushed 

 anteriorly ])v the growtli (jf the succeedirjg segments. About two weeks 

 before the hatching of the embryo the brain proper liecomes divided 

 into two transverse ])ortions by a constriction (fig. 69). 'J'hiis the 

 brain of Jjimulus jnay be divided into four i)arts — the <'*anoliou of the 

 median eyes, the ganglion of the lateral eyes, and the antérieur and 

 posterior portions of the brain proper. 'J'hese four i)arts are arranged 

 in three transverse rows. 'Jlie anterior row is occupied bv tlie two 

 pairs of tlie <.)ptic ganglia, the middle row by the anterior ]K)rtion of 

 the brain pi'oper, and the posteri<jr row by the posterioi" p(»rti<jn of the 

 brain proper (tig. 69). 



Though the posterior p(jrtion of the l)rain pi-nper is verv early 

 (earlier than any otlier ])art of the nervous system) cut (jif from the 

 e))iblast, and the iil)rous conunissure is de\ eloped between the I'ight 

 and lei't sitles (figs. 45, 5J, 69), its anterior ])ortion as well as the two 

 ]>airs of tlie o])tic ganglia are for a long time not separated from the 

 epiblast (fig. 56). 



The ventral nci've-cord consists at first of one continuous I»»noi- 

 tudinal thickening of the epiblast (tigs. 1:^, 30, o.")). The thickening 

 is at this time distinguished by the pe«;uHar cell-groups aln^adv ]neh- 

 tioned. At altout the stage of fig. 13, the thickening is divided into 

 ganglia, as the cells conij)osing it accumulate into a mass in CAery 

 segment. A ])air of sudi ganglia may be found in eacli of the 

 segments (jf the first and se\enth appendages as in the other segments. 

 I he ganglia of the first and tlie second a [>j)rndage-bearing segments 

 form the cireum(i'so])hageal ring (figs. 51 aiid 69). 



Lying between tlic two laicral mcjubei-s of the L:-ani;-|ia tliere are 



