THE NEURAL SINUS, NEUROGLIA AND CANALIS CENTRALIS. 



331 



appearance in the different segments, the ones between the fifth and sixth neuro- 

 meres being the largest. In stage A', one of these thickenings forms a disc-like 

 mass of dense tissue, with deeply stained nuclei, lying on the ventral surface of the 

 sternum beneath the sixth interganglionic space of the thorax. (Fig. 229, m.l.ch.) 

 This body is the merochord. It appears to be merely an isolated local enlarge- 

 ment of the thoracic portion of the lemmatochord. 



C. Development of the Neural Sinus, Neuroglia and Canalis Centralis. 

 -The middle cord groove in the scorpion develops in essentially the same way as 

 in Acilius. 



From the walls of the intraganglionic portions are formed a. ganglion cells; 



)' ^%, 8 a0@ . A^^a'.f- ra^s>|e\ _ SSSQfi 



-~^- to_.;-!o ; v<a> era^-i"'^- 



-p.S.O. 



^-r 



/. tr-* . -^;jllls|J> 



' j*-^v~ 1 ' -fc- 1-* 



l-lTtl-rL.p. "' -'inl.' 



FIG. 227. Scorpion embryo. No. i, Section of an abdominal segment in front of the primitive streak, stage A ; 

 No. 2, abdominal neuromere, showing the breaking up of the medullary plate into primitive sense organs, stage C; 

 No. 3, abdominal region, between two neuromeres, stage E\ No, 4, abdominal region through the middle of a neuro- 

 mere, stage E. 



b. the epithelium of a central canal; c. neuroglia cells. From the interganglionic 

 portions arise a. transient nerve fibers of the median nerve connectives; b. walls 

 of the neural sinus; c. blood corpuscles. 



In the adult scorpion, the walls of the neural sinus consist of an endothelial 

 layer of clear cells, often in sharp contrast with the enclosed coagulum. They 

 pass without perceptible break into confused masses of tissue, and these into 

 blood corpuscles, w r hich seems to indicate that the latter are formed from the walls 

 of the sinus during adult life. 



It is difficult to define exactly where the sinus terminates anteriorly. In the 

 thoracic region of the adult, the interganglionic spaces may be quite large and 

 filled with the gelatinous substance and free cells, but the spaces do not seem to 

 communicate with each other freely or to be directly connected with the abdominal 

 sinus. 



