THE REGION OF THE SPINAL CORD 407 
removed from the surface and caused to assume the deeper position, 
as seen in the later embryo, is perfectly clear and uniform in all the 
vertebrate groups. The diagrams at the end of van Wijhe’s paper, 
which I reproduce here, illustrate the process which takes place. At 
first the myotome (Fig. 159, A) is confined to the dorsal region on 
each side of the spinal cord and notochord. Then (Fig. 159, B) it 
separates from the rest of the somite and commences to extend ven- 
trally, thus covering over the pronephros and its duct, until finally 
(Fig. 159, C) it reaches the mid-ventral line on each side, and the 
foundations of the great somatic body-muscles are finally laid. 
In order, therefore, to understand how the obliteration of the 
appendages took place, we must first find out what is the past history 
of the myotomes. Why are they confined at first to the dorsal region 
of the body, and extend afterwards to the ventral region, forcing by 
their growth an organ that was originally external in situation to 
become internal ? 
In the original discussion at Cambridge, I was accused of violating 
the important principle that in phylogeny we must look at the most 
elementary of the animals whose ancestors we seek, and was told 
that the lowest vertebrate was Amphioxus, not Ammoccetes; that 
therefore any argument as to the origin of vertebrates must proceed 
from the consideration of the former and not the latter animal. My 
reply was then, and is still, that I was considering the cranial region 
in the first place, and that therefore it was necessary to take the 
lowest vertebrate which possessed cranial nerves and sense-organs of 
a distinctly vertebrate character, a criterion evidently not possessed 
by Amphioxus. Such argument does not apply to the spinal region, 
so that, now that I have left the cranial region and am considering 
the spinal, I entirely agree with my critics that Amphioxus is likely 
to afford valuable help, and ought to be taken into consideration as 
well as Ammoccetes. The distinction between the value of the spinal 
(including respiratory) and cranial regions of Amphioxus for drawing 
phylogenetic conclusions is recognized by Boveri, who says that, in 
his opinion, “ Amphioxus shows simplicity and undifferentiation 
rather than degeneration. If truly Amphioxus is somewhat degene-° 
rated, then it is so in its prehensile and masticatory apparatus, its 
sense organs, and perhaps its locomotor organs, owing to its method 
of living.” 
Hatschek describes in Amphioxus how the ccelom splits into a 
