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 
cesophagus. 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— 
