THE NEURAL SINUS, NEUROGLIA AND CANALIS CENTRALIS. 



333 



cord is distinctly tubular. (Fig. 22^.) Owing to the crowding of the thoracic 

 neuromeres, these middle cord tubes almost unite above the interganglionic spaces 

 thus forming a nearly continuous canal. 



In the next stage, sections through the middle of each abdominal neuromere 

 show that the invaginated portion of the median furrow is losing its central cavity, 

 and now lies in the heart of the neuromere as a great cluster of cells difficult to 

 distinguish from the surrounding nerve cells. At the anterior and posterior ends 

 of the neuromere, the tissues of the middle cord are continuous with the neuroglia 

 layer separating the medulla from the cortex. (Fig. 228', i.n.l^ 



ni.lek. ^^ 

 l.Uh, CTa. IcK' 



Fig. 229. — Scorpion embryos, stage K, showing the later stages in the development of the middle cord, neural 

 canal, neural blood-vessel, genital cells and lemmatochord. No. i. Section through the posterior vagus neuromere 

 showing the transverse commissural fibers above and below the remnants of the middle cord (neural canal) ; No. 2 

 section through the anterior vagus neuromere, showing median and lateral divisions of the lemmatochord; No. j, 

 section just in front of the sixth thoracic neuromere, showing the local enlargement of the lemmatochord; No. 4, 

 section between the second and third vagus neuromeres, showing the enlarged lemmatochord and the anterior end 

 of the germ-cell cord; No. 5, section through the middle of the third free abdominal neuromere; No. 6, section about 

 midway between the third and fourth vagus neuromeres; No. 7, section through the anterior margin of the fourth 

 vagus neuromere; No. 8, section through the posterior portion of the third free abdominal neuromere. 



The nterganglionic spaces in the thorax contain, besides a few scattered 

 nuclei, a loose fibrous cord that appears to run from one neuromere to another. 

 It probably represents the remnants of a median nerve. 



In stages H and K, remarkable changes have taken place. All the inter- 

 ganglionic spaces are crowded with rounded cells. They fill the interganglionic 

 spaces and push their way forward and backward under the ganglia till they form 

 a continuous cord. (Fig. 229'"'.) They are everywhere shut off from the sur- 

 rounding tissues by the inner neurilemma and by the basement membrane. 

 At the anterior and posterior ends of each neuromere they are continuous with 

 the intraganglionic portion of the middle cord. The latter is now reduced to a 

 small but well defined cord of cells in the middle of each neuromere just above 

 the medulla. (Fig. 229''~^) It is probable that the cells filling the interspaces 

 arose from a rapid proliferation of the ends of the ganglionic portion of the 



