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 coelom splits into a 



