612 MARGARET ROBlNSON. 



Lave a rather wavy outline all through their lengthy but after 

 much careful observation I was able to make out in each cord 

 a dip or incurving, surrounded by a semicircular thickening, 

 of the cord. This incurving lies about half-way between the 

 imperfect closing of the nerve-ring and the second "inter- 

 segmental furrow" — i. e., in the middle of the second post- 

 oral segment. This appearance is repeated in the mandibular 

 segment. These semicircular, thickened incurvings of the 

 cords must be the rudimentary tritocerebral and mandibular 

 ganglia. 



One cannot help being struck by the well-marked double- 

 ness of the nervous system shown here. In Scolopendra, 

 according to Heymons, the median unpaired ectodermal 

 thickening (which he calls the archicerebrum) is the part of 

 the nervous system which appears first. Later on it forms a 

 junction between the two halves of his syncerebrum, ultimately 

 becoming fused with them. These two halves, therefore, are 

 never separated from each other by non-nervous tissue. But 

 Heathcote (1886), if I understand him rightly, records a 

 double origin for the nervous system of Jul us, and 

 Metschnikoff (1874), with more clearness, speaks of the first 

 rudiment of the brain in Strongylosoma as consisting of 

 two " Scheitelplatten." 



I do not believe that the tritocerebral segment has, up till 

 now, been observed in a Diplopod embryo. It is, however, 

 not surprising to find it, since St. Kemy (1890) has described 

 a well-mai'ked tritocerebruin in the brains of Jul us and 

 Glomeris (adult). 



Before leaving the future head, a word or two must be said 

 about the procephalic lobes. They resemble those found in 

 embryonic insects, arachnids, etc., and those figured by 

 Heymons (1897) in the embryo of Grlomeris. Their size 

 would seem to me to almost preclude the possibility of find- 

 ing a separate prtc-antcnnary segment here. 



Behind the tritocerebral segmentlies that of the mandibles, 

 and following that two segments bearing maxilla). 1 have not 

 been able to demonstrate the ganglia belonging to these last 



