336 THE MIDDLE CORD, THE LEMMATOCHORD AND THE NOTOCHORD. 



drical, unsegmented cord, lying on the haemal side of the nervous system, and 

 enclosing the remnants of the median nerve. 



3. In the scorpion, the interganglionic pits of the median groove, in the 

 abdominal region, give rise to groups of cells, some of which become free blood 

 corpuscles, others neuroglia cells, and others form the walls of separate sinuses, 

 comparable with the neurilemma of a segment of a median nerve. The inter- 

 ganglionic spaces finally extend forward and backward on the haemal side of 

 the neuromeres, forming a continuous neural sinus. 



4. The bothroidal cord, or lemmatochord, appears to develop from the primi- 

 tive streak. Later it unites with the wall of the neural sinus. It probably 

 represents the enlarged neurilemma of the lateral and median cords, and to- 

 gether with the neural sinus, corresponds approximately to the lemmatochord 

 of lepidoptera. The merochord represents a local enlargement of the lemmato- 

 chord in the sixth thoracic segment. 



5. With the crowding together of neuromeres, the interganglionic and in- 

 traganglionic segments of the middle cord tend to unite and to form two con- 

 tinuous cords, or canals, which stand on different levels, and which have different 

 functions. One forms an internal cord, or canal, which lies in the central portion 

 of the neuromeres between the neural and haemal cross commissures; it gives rise 

 to neuroglia cells, nerve cells, and the epithelium of the canal. The other forms 

 an external cord, strengthened by a heavy investment of neurilemma, and serves 

 for the attachment of muscles. Both cords may contain a longitudinal canal, 

 and each canal may open into the other through segmentally arranged openings 

 between the cross commissures of successive neuromeres. These conditions are 

 represented diagrammatically in Fig. 231. 



6. These two cords are represented in vertebrates by the epithelium of 

 the canalis centralis, with its adjacent neuroglia tissue, and by the notochord. 

 The embryonic origin of both cords, in vertebrates as in arthropods, may be 

 traced to the growing apex of the embryo, and the method of growth of these organs 

 appears to be essentially the same in both classes. 



Whether the median groove is differentiated into its component parts grad- 

 ually, so that all the intermediate stages are seen at the same time, arranged 

 in superficial linear order from one end of the embryo to the other, as in the 

 scorpion; or whether the differentiation takes place rapidly, at some deep-lying 

 point in the primitive streak, as for example in Amphioxus, is merely an ontogenetic 

 variation due to the amount of detail in the process of recapitulation and to the 

 relative time at which the details appear. 



In the vertebrates, the early stages in the differentiation of the middle cord 

 are passed through rapidly, forcing the anterior end of the notochord below 

 the surface as fast as it is formed at the primitive streak. Its primitive relation 

 to the tissues lining the floor of the neural canal is indicated by the temporary 

 communications that obtain between the cavity of the notochord and that of the 

 canalis centralis at the posterior end of vertebrate embryos. 



