33 2 



THE MIDDLE CORD, THE LEMMATOCHORD AND THE NOTOCHORD. 



In surface views of young embryos, the lateral cords are seen to be separated 

 by a continuous shallow groove. (Fig. 15, A.) As the cords break up into neuro- 

 meres, the median groove narrows and deepens and becomes distinctly divided 

 into a succession of oval pits, one between each half neuromere. (Fig. 15, B.) 

 The anterior pits become more distinct for a while; their walls are then incor- 

 porated into the body of the neuromere at the points where the transverse com- 

 missures are formed. The posterior pits are formed between the future longi- 

 tudinal connectives; they gradually flatten out and become less distinct in sur- 

 face views. Sections from this stage show that the ectoderm of the posterior 

 pits thins out and draws away from the underlying basement membrane, forming 

 clear areas, or interganglionic spaces. (Fig. 226 4 , i.g.s.) They may contain a 



c.c 



U ,* faaSSss -^L"- 



FIG. 228. Scorpion embryo, stage G; cross-sections of the nerve cord, illustrating the development of the middle 

 cord, neuroglia, neural blood-vessel, lemmatochord, etc. No. i, Section through the posterior margin of the third 

 vagus neuromere (comb segment) ; No. 2, section through the middle of the third abdominal interganglionic space, 

 or the one next behind the comb neuromere; No. 3, section through the posterior margin of the same space; No. 4, 

 section through the interganglionic space between two thoracic neuromeres, showing the continuity of the outer 

 portion of the middle cord from one neuromere to the other, and also the deeper lying cells derived from the inner 

 portion of the middle cord; No. 5, section between the fifth and sixth thoracic neuromeres; No. 6, section through 

 the middle of the fourth thoracic neuromere; No. 7, section through the middle of the second thoracic neuromere, 

 showing the middle cord as a prominent mass of cells on the bottom of the neural canal. The middle cord is not 

 connected with the ectoderm at this point. No. 8, Section through the space between the second and third thor- 

 acic neuromeres. 



finely granular substance, a few fibers, and an occasional nucleus, the latter lying 

 just above the basement membrane, or among the fibers extending downward 

 from the roof. In stages E and F, the abdominal spaces contain a few free cells 

 which are undoubtedly blood corpuscles. As the spaces at this period are com- 

 pletely closed, the blood corpuscles were evidently formed by a modification of 

 the ectodermic cells of the middle cord. 



In stage F, a few light coloied cells appear on the periphery of the interspaces 

 that mark the beginning of the neuroglia, or the inner neurilemma. (Fig. 22j,i.n.L) 



In stage F, the spaces are small and shut off from the ectoderm by the union 

 of the outer parts of the lateral nerve cords. (Fig. 227 3 .) The intraganglionic 



