334 



THE MIDDLE CORD, THE LEMMATOCHORD AND THE NOTOCHORD. 



middle cord, as well as from the division of the scattering cells seen in these 

 spaces at an earlier period. The same kind of cells arise in those parts of the 

 middle cord that remain united with the ectoderm, forming there masses of cells 

 continuous with those in the underlying interspaces. (Fig. 229^ b.c.) The 

 wedges are best developed just back of each free abdominal ganglion; their 

 central cells become free blood corpuscles, and the walls form the vertical 

 vessels arising from the neural sinus. 



Sections through the vagus neuromeres show that the cells filling the in- 

 terganglionic spaces are united with those in the spaces in front and behind 

 by two cell cords; one is the intraganglionic portion of the middle cord, and lies 

 in the medulla between the neural and heemal commissures; the other consists 

 of cells that have pushed their way beneath the medulla. (Fig. 229^, m.ch, Ic.h.) 



Fig. 230. — Scorpion embryo, about ready to hatch. No. i, Section through the space between the fifth and 

 sixth thoracic neuromeres; No. 2, section through space between sixth and seventh thoracic neuromeres; No. 3, 

 section through posterior portion of the second vagus neuromere; No. 4, section through the first free abdominal 

 neuromere; No. 5, section between the second and third free .abdominal neuromeres; No. 6, section through the 

 posterior margin of the third vagus neuromere; No. 7, section just back of the third free abdominal neuromere; 

 No. 8, section between the second and third-thoracic neuromeres, showing the remnants of the anterior end of the 

 middle cord. 



Soon after stage H, the cell cord that filled the interganglionic spaces, and 

 that extended beneath the medulla of each ganglion, is replaced by a thin-walled 

 tube, the neural sinus. 



III. Middle Cord of Limulus. 



It is difficult to follow the middle cord in Limulus. In the adult it lies inside 

 the tough outer sheath of the nerve cord, and consists of irregular masses of matted 

 tissues resembling that in the bothroidal cord of the scorpion. (Figs. 55, 67, 68.) 

 It is arranged in two main lateral cords, one on either side of the neuromere, l.l.ch. 

 In the vagus region, a. conspicuous median mass is present. (Fig. 55, m.lch.) 

 The lemmatochord tissue is continuous, by means of fine fibers, with the neuroglia 

 that everywhere permeates the nerve cord and forms an envelop for the ganglion 

 cells and the bundles of nerve fibers. 



