ON THE ORIGIN OF VERTEBRATES FROM ARACHNIDS. 377 



EXPLANATION OF PLATES XXIII and XXIV. 



Illustrating Mr. William Patten^s paper " On the Origin of 

 Vertebrates from Arachnids." 



a. n. Abdominal neuromeres. br. Brain, c. g. Coxal glands or nephridia. 

 ch. Chelae. cJil. Chelicerse, c. s. o^. Indicated position of coxal sense-organs. 

 e. s. Outline of optico-ganglionic sac. f. b. ■= s. I. Frontal or semilunar 

 lobe, g.m.c. Ganglionic portion of median furrow, g.v^-^. Ganglionic 

 invagination. i. m. e. luterganglionic portion of median furrow. /. 6'-^. 

 Lung-books. /. e. Lateral eyes. m. Mouth, m. e. Median eye. mxl. 

 Maxillaria. n.ph. Nephridia. n.c. = sp.g. Neural crest or spinal ganglion 

 "Anlage." op. g^-^. Optic ganglia, op.p^-^. Optic plates, p^-^. Walking 

 legs. s. I. Semilunar lobe. s. o^. Sense-organs near joints of legs. s. d^. 

 Scattered sense-organs, sp, n. Spinal nerve, s. s. o. Segmental sense-organs. 

 st. n. Anlage of stomodseal nerves, t. n--^. Thoracic neuromeres. v. w^-''. 

 Vagus neuromeres. v. p^-*. Vagus appendages. 



PLATE XXIII. 



Fig. 1. — Stage b : surface view of embryo Scorpion removed from the 

 yolk. Observe the forward position of the mouth, and the absence of nervous 

 or other tissue immediately in front of it ; the absence of abdominal append- 

 ages ; the post-oral position of the cheliceraj ; the divergence of the abdominal 

 nerve-cords; the segmental sense-organs, s.s.o.; the absence of the eyes, 

 and of the rudimentary optic plate ; the innumerable sense-organs from 

 which the nervous system arises ; the large sense-organs, n. c. ^ sp. g., which 

 later form an incipient " neural crest," and from which the spinal ganglia 

 develop ; the "anlage" of the optic ganglia, op.g., and of the brain, be, which 

 is not yet segmented. (Obj., oc. 2.) 



Fig. 2. — Stage c : surface view of detached embryo of Scorpion, show- 

 ing the three brain-segments ; the beginning of the second and third gan- 

 glionic invagination, g. v". and g. v^. ; the paired median sense-organ-like in- 

 vagination in the brain region, those opposite the fourth neuromere giving rise 

 to the ganglia of the stomodseal nerves ; the continuity of the paired pits with 

 the median furrow ; the great frontal lobe,/! b., produced by the invagination 

 of the first brain metamere, and possibly made up in part of an Annelid pre-oral 

 lobe (?) ; the large optic ganglion of the second and third brain metameres, 

 op.g-. and op.g^.; the nephridia-like cell-coils in the basal portion of each 



