1897.] amphioxus and balanoglossus. 311 



Now these sacs practically give rise to all that is contained 

 between the skin and gut of Amphioxus with the exception of 

 the notochord and the nervous tube. In the paper referred to 

 I compared (3) to the head ccelom of Balanoglossus, and 

 (2) and (1) to the collar and trunk cceloms respectively of that 

 animal. 



It is a priori improbable that any form exactly like the 

 ancestor of a living animal has survived to this day. For ex 

 hypothesi such a type of structure has been found to be unsuited 

 to the environment and has been modified by the struggle for 

 existence. If, however, whilst some of the ancestors of Vertebrates 

 were undergoing evolution into higher forms under the stress of 

 competition, others betook themselves to situations, like the 

 mud, where they could escape the brunt of the struggle, such 

 animals would preserve the general level of ancestral structure 

 in a more or less degenerate form with special adaptations to a 

 burrowing life, and this we suppose Balanoglossus to have done. 



In the higher Vertebrates the homologue of the head cavity 

 has been detected in that bilobed head cavity which in the 

 embryos of fish underlies the eyes, and from whose walls the 

 majority of the eye muscles are formed, but the homologue of 

 the collar cavity has not yet been sought for and naturally has not 

 yet been discovered. 



In my former communication on this subject I alluded to a 

 paper by Lwoff on the development of the germinal layers in 

 Amphioxus. In this paper Lwoff not only maintains that the 

 ccelom or body cavity does not originate from the alimentary 

 canal, but that the whole mesoderm and the notochord itself 

 originate from an ingrowth of the ectoderm. If this were true 

 any possible comparison of Balanoglossus and Amphioxus would 

 be rendered impossible ; it would be a result of the most far- 

 reaching character. It may be remarked that it used formerly 

 in many cases to be asserted that the ectoderm gave rise to the 

 mesoderm where later renewed research has shown that the 

 mesoderm arose from the alimentary tube. 



I have now further evidence to offer in favour of this view. 

 As I have said, the trunk ccelom becomes divided into separate 

 muscular segments, these later become differentiated into a dorsal 

 muscular and a ventral thin-walled portion ; and then the dorsal 

 portions become completely separated from the ventral parts, 

 and simultaneously the latter become fused with one another to 

 form two longitudinal ccelomic canals. 



What I have designated as the collar coelom undergoes also 

 differentiation into a dorsal muscular and a ventral thin-walled 

 portion. The two do not however for a long time become con- 

 stricted off from one another; and even when this occurs the 

 VOL. IX. PT. VI, 25 



