330 



THE MIDDLE CORD, THE LEMMATOCHORD AND THE NOTOCHORD. 



length into two layers, the under one being the anlage of the sexual organs, g.c., 

 the upper one, the lemmatochord, l.ch. In a series of cross-sections of this stage, 

 beginning at the posterior end of the abdomen, the lemmatochord is first seen just 

 back of the sixth abdominal ganglion, as a dark lenticular thickening. (Fig. 230 1 .) 

 The cord has the same flattened appearance in all the sections until we reach the 

 vagus neuromeres, when it suddenly enlarges and assumes more of its future 

 appearance. (Fig. 229, 4 " 7 .) The anterior end of the lemmatochord is seen 

 between the second and third vagus neuromeres. (Fig. 22g 2 .) 



From this stage up to the time of hatching, the genital cells gradually sepa- 

 rate from the lemmatochord, and the latter separates from the neural sinus, except 

 at certain places where it remains permanently attached to the neurilemma of the 





FIG. 226. Embryo scorpion (Buthus carolinianus). No. i, 2, 3, Sections through the posterior end of the 

 embryo, stage A; No. 4, 5, 6, 7, sections through the interganglionic spaces of the nerve cord, stage B.C; No. 8, 

 section through the middle of a terminal neuromere, stage C. 



middle cord. By the time the body pigment is well developed, the lemmato- 

 chord of the first free abdominal ganglion appears as in Fig. 230*. In this 

 figure we can see indications of the passageway into the sinus. The lemmato- 

 chord, just behind the third vagus neuromere, is reduced to a slender fiber. (Fig. 

 230 6 .) At the posterior end of the abdomen it decreases in size and disappears, 

 apparently running directly into the thickened wall of the neural sinus. (Fig. 



2 3 5 .) 



In embryos just hatched we may obtain good surface views of the lemmato- 

 chord by dissecting out the entire nervous system. (Fig. 71.) 



In half grown scorpions the lemmatochord is in about the same condition as 

 in the adult. 



Merochord. The lemmatochord tissue extends into the thoracic region, giving 

 rise to the merochord and to two adjacent, parallel cords of dense connective tissue. 

 (Fig. 71.) The first traces of these structures are a few isolated cells, lying be- 

 neath the interganglionic spaces of the thoracic and the first two or three ab- 

 dominal segments. They form lenticular thickenings, which vary in size and 



