VERTEBRATES PROM A ORUSTACEAN-LIKE ANCESTOR. 395 



and the limits of which are clearly marked out by the layers 

 of blackened fat-cells^ extends close to the ventral surface^ so 

 that the nerve-matter with its groups of ganglion-cells giving 

 origin to the segmental cranial nerves is situated on each 

 side of, and ventrally to, the original large bag ; in other 

 words, a small ventral portion of the cephalic stomach has been 

 nipped in this region by the growth of the two laterally situated 

 masses formed by the upper ganglia of the ventral chain. By 

 this lateral compression the original cavity has become first 

 converted into a vertical slit, and then by the fatty degenera- 

 tion of its cell-walls into an interlacement of fibres, known 

 in anatomy by the name of the raphe. 



This formation of a raphe or seam by the lateral compres- 

 sion of two nerve-masses can be followed up to the infundibu- 

 lum; in other words, not only the upper ganglia of the ventral 

 chain, but also the infra-oesophageal ganglia, have compressed 

 a small ventral portion of the cephalic stomach, thus forming 

 the raphe of the aqueduct. In this region, however (fig. 

 3, PI. XXV, r. aq.), the raphe no longer extends to the ventral 

 surface of the brain, but, on the contrary, the ventral surface is 

 occupied by a tube, the sides of which have been compressed as 

 already mentioned, not so as to form a vertical slit, but, on the 

 contrary, so as to form a horizontal slit. In other words, the 

 growth dorsalwards of the bilateral infra-oesophageal mass has 

 nipped and compressed a ventral fold of the cephalic stomach, 

 with the result of forming a vertical slit, which ultimately be- 

 comes the raphe of the aqueduct; and at the same time the 

 increase in size of the whole infra-oesophageal nervous mass has 

 compressed (as already explained) the oesophagus which lies 

 between it and the wall of the cranial cavity, with the natural 

 result of causing the lumen of the oesophageal tube to take 

 the form of a horizontal and not a vertical slit. Also we see 

 that the presence and position of the horizontally compressed 

 oesophagus in the infundibular projection is the reason why 

 the raphe comes to the ventral surface in the middle line only 

 in the epichordal portion of the brain. 



The appearance, then, of an interlacement of fibres in the 



