THE REGION OF THE SPINAL CORD 405 



which has played an important part in the formation of the verte- 

 brate, viz. that the meristic variation by which the spinal region of 

 even the lowest of existing vertebrates has been formed, has largely 

 taken place in the vertebrate phylum itself, and that such changes 

 must be eliminated before we can picture to ourselves the pre-verte- 

 brate condition. As an example, I may mention the remarkable 

 repetition of similar segments pictured by Bashford Dean in Bdello- 

 stoma. Such repetition leads to passive lengthening of such parts 

 as are already formed but are not meristically repeated : such are the 

 notochord, the vertebrate intestine, the canal of the spinal cord, and 

 possibly the lateral line nerve. The fuller discussion of this point 

 means the discussion of the formation of the vertebrate alimentary 

 canal ; I will therefore leave it until I come to that part of my 

 subject, and only say here that the evidence seems to me to point to 

 the conclusion that at the time when the vertebrate was formed, the 

 respiratory and cloacal regions were very close together, the whole of 

 the metasoma being represented by the region of the pronephros 

 alone. 



Here, as always, the evidence of Ammoccetes tends to give 

 definiteness to our conceptions, for Wheeler points out that up to a 

 length of 7 mm. the pronephros only is formed ; there is no sigu of 

 the more posteriorly formed mesonephros. Now we know, as pointed 

 out in Chapter VI., p. 228, this is the time of Kupffer's larval stage 

 of Ammoccetes. This is the period during which the invertebrate 

 stage is indicated in the ontogeny, so that, in accordance with all 

 that has gone before, this means that the metasoma of the inverte- 

 brate ancestor was confined to the region of the pronephros. 



Again, take Shipley's account of the development of Petromyzon. 

 He says — 



" The alimentary canal behind the branchial region may be 

 divided into three sections. Langerhans has termed these the stomach, 

 midgut, and hindgut, but as the most anterior of these is the narrowest 

 part of the whole intestine, it would, perhaps, be better to call it 

 oesophagus. This part of the alimentary canal lies entirely in front 

 of the yolk, and is, with the anterior region, which subsequently 

 bears the gills, raised from the rest of the egg when the head is 

 folded off. It is supported by a dorsal mesentery, on each side of 

 which lies the head-kidney (pronephros)." 



Further on he says — 



