THE LEMMATOCHORD OF LEPIDOPTERA. 



327 



angular in cross-section. Tlie two lateral bands finally crowd the median one 

 inward, and unite with it to form a single cylindrical cord, in which the arrange- 

 ment of the chorda cells may still indicate the separate origin of peripheral and 

 axial cells. (Fig. 224, A. s.mn. and s.l.c.) Meantime the median nerve has dis- 

 appeared, or at least no traces of it can be seen, except where it rises to the surface 

 of the chorda to enter the ganglion, m.n. 





Fig. 223. — Sections through the abdominal nerve cord of Cecropia, early pupal stage, showing the greatly 

 thickened sheaths of the median and lateral nerve cords. A, Midway between two neuromeres; J5, near the pos- 

 terior side of a neuromere. 



At the close of the metamorphosis, the cord has become irregularly oval, the 

 walls of the chorda cells have disappeared, and theii nuclei have become iriegular 

 in shape and size. (Fig. 224, B.) As the chorda assumes its final form, a thick 



~ Fig. 224, — Nerve cords and lemmatochord of Cecropia. A , Late pupal stage showing the fully formed lemmato- 

 chord, derived from the condensed sheaths of the median and lateral cords; also remnants of the median nerve; B, 

 adult Cecropia. The tissue of the lemmatochord has undergone degenerative changes, and at this point is invaded 

 by the ends of the attached muscle cells. 



hyaline sheath forms around it, except along its haemal side where the muscle cells 

 are attached. 



At the beginning of the pupal period the thickened neurilemma on the neural 

 side of the lateral cords becomes inflated with a network of trachea, tr., which 

 later disappears, leaving the cords again surrounded by a thin double layered 

 membrane. 



The chorda muscles appear at the close of the larval period, just in front of 



