324 THE MIDDLE CORD, THE LEMMATOCHORD AND THE NOTOCHORD. 



brate and invertebrate notochord are sufficiently clear. The beginning of the 

 notochord may be recognized in practically all segmented invertebrates, as the 

 so-called middle cord, or median nerve, and in its derivative, the lemmatochord. 

 This structure forms a fundamental part of the body in all segmented inverte- 

 brates. It undergoes many modifications, but its location, function, mode of 

 growth, and its development, are in all cases essentially the same, and leave 

 no reasonable doubt that it is indeed the long looked for notochord of the in- 

 vertebrates. 



I. The Middle Cord of Insects. 



Acilius (Figs. 221, 222).— The middle cord arises at a very early em- 

 bryonic stage as a median longitudinal groove that extends from the posterior 

 margin of the stomodaeum to the posterior end of the body. It should not be 

 confused with the so-called primitive groove of arthropods, or with the neural 

 groove of vertebrates. 



Fig. 221. — The second pedal neuromere of an embryo of Acilius; serial sections showing the development of the 

 middle cord, and its relation to the cross commissures," inner neurilemma, or neuroglia, and median nerve. 



The walls of the groove, in the interganglionic portions, give rise either to 

 thickenings to which muscles are attached (thoracic region) or to the median 

 nerve (abdominal region). The walls of the intra-ganglionic groove give rise 

 to nen'e cells that form a part of the ganglia, and to the neuroglia. 



The early conditions are shown in a series of sections of the nerve cord of 

 Acilius opposite the second pair of legs, "during the formation of the dorsal oigan. 

 (Fig. 221.) 



The groove deepens in front of the second pedal neuromere and its nuclei 

 become darker colored, A^. In the middle of the neuromere, the outer walls of 

 the groove have almost disappeared, while the floor forms a solid triangular block 

 of cells overlying the cross commissures, A'^. A few nuclei, like those in the 

 middle cord, lie underneath the lateral ends of the cross commissures, iiil. On 



