350 WILLIAM PATTEN. 



VII. The Median Furrow (or Mittelstrang of Hatschek) 

 AND THE NoTOCHORD. — The Ventral ncrvous system of Arthro- 

 pods consists of three longitudinal cords (five in Limulus), 

 two lateral and one median. The latter, in its indifferentiated 

 state, is represented by the so-called " sympathetic^' or " median 

 nerve.'' 



In Acilius, between the ganglionic swellings of the lateral 

 cords the median furrow is almost tubular and somewhat 

 swollen, forming what may be regarded as the ganglia of 

 the median nerve. In the abdomen the interganglionic 

 portions of the median furrow give rise to the median nerve 

 proper, which is therefore merely a longitudinal connective, 

 comparable with those of the lateral cords. 



In the thoracic region the interganglionic parts of the 

 furrow produce great ectodermic thickenings (the furcse), to 

 which muscles are attached. 



In Scorpio, the median furrow, in the centre of each neuro- 

 mere forms a deep pit, the thick walls of which, epithelium 

 and all, are converted into ganglion-cells (Pis. XXIII and 

 XXIV, and Fig. 2, a, c, and e). By the crowding together of 

 the thoracic neuromeres the pits in some places unite, forming 

 a temporary central canal. 



Between successive abdominal neuromeres the median 

 furrow proliferates inwards, producing solid spindle-shaped 

 clusters of cells, which grow forward and backward until they 

 meet, forming beneath the nervous system a continuous 

 longitudinal cord (Fig. 2, g, i. m. c). Owing to its method of 

 formation, the latter is for some time segmentally swollen. In 

 embryos about to hatch, the cord is hollowed out, and forms 

 the ''spinal artery." The whole cord is sometimes filled 

 with large vesicular cells with small nuclei (Fig. 2, c), and in 

 half-grown specimens the wall of the artery may be extremely 

 thick and hyaline (d). Thus the whole organ presents a strik- 

 ing resemblance to a Vertebrate notochord. 



At the anterior end of the embryo, beneath the vagus neuro- 

 meres, the interganglionic part of the median furrow forms a 

 great solid ball of tissue, composed of a confused mixture of 



