156 DEVELOPMENT OF AMPHIOXUS. 



The phenomenon of asymmetry manifests itself in the 

 larva of Amphioxus under several very different aspects, 

 and is occasioned by various causes. For convenience we 

 may classify the forms of asymmetry which we have to 

 consider under three main divisions, according to the type 

 of organs involved. 



1. Median Asymmetry: — This relates to such structures 

 as lie normally in the middle line, whether dorsal or ven- 

 tral, but which have been mechanically or correlatively dis- 

 placed from their primitive position by the differential 

 growth of neighbouring parts. Such are the olfactory pit 

 and neuropore, the anus, the mouth, and the endostyle. All 

 these are essentially and primordially median and unpaired 

 structures. We have already dealt with the neuropore 

 and anus, while the mouth and endostyle will be con- 

 sidered below. 



2. Bilateral Asymmetry. — This refers to the alternation 

 of paired structures, such as myotomes, spinal nerves, gill- 

 slits, and gonads, which we have already noted in the adult 

 organisation. Primarily the organ of one side lies opposite 

 to its antimere of the other side. By a secondary displace- 

 ment it comes to alternate with it.* 



3. Unilateral Asymmetry. — Next to the asymmetrical 

 mouth, this is perhaps the most striking form of asym- 

 metry which the larva of Amphioxus exhibits. It relates 

 to those structures which belong to the category of paired 

 organs, but which, in the course of the larval development, 

 appear unpaired on one side of the body. Such are the 



* When the myoccelomic pouches first appear in the embryo they are 

 placed symmetrically. At an early stage, however (see Fig. 63 j9), the alter- 

 nation sets in. This involves such later-appearing structures as the spina! 

 nerves and gonads, so that they alternate from the time of their first origin. 

 The alternation of the gill-slits would seem to be independent of that of the 

 myotomes. 



