NOTES. !77 



development of the left half of the oral hood and the left meta- 

 pleur has been secondarily brought about. 



10. (p. 150.) The study of transverse sections has led me to 

 the conclusion that the backward extension of the endostyle is 

 effected by interstitial growth, and not by the conversion of the 

 cells which form the primary floor of the pharynx into endostylar 

 elements. These cells are probably disintegrated and absorbed 

 by the endostyle as it grows backward. 



11. (p. 153.) For a comparison between the perigonadial 

 cavities of Amphioxus and the mesonephric tubules of the 

 craniates the reader should consult Boveri's original memoirs. 

 (See bibliography.) 



12. (p. 159.) The following definition of the so-called bio- 

 genetic law of recapitulation (Haeckel's biogentisches Grund- 

 gesetz) will explain the meaning of Haeckel's terms " cenogenesis " 

 and " palingenesis." According to this law : The development of 

 the individual (ontogeny} is a compressed summary of the gradual 

 modifications which have resulted in the evolution of the species, 

 or type (phytogeny = Stammesgeschichte) ; this recapitulation 

 (summary, or Auszug) of the phylogenetic stages in the ontogeny 

 is the more perfect according as the ancestral development 

 (Palingenesis, Auszugsentwicklung) has been the less disturbed 

 or falsified through secondary or " recent " adaptation (ceno- 

 genesis, Storungsentwickelung) of the embryo or larva to a new 

 environment. 



13. (p. 162.) The explanation of the asymmetry of the larva 

 of Amphioxus given in the text was first suggested by me in 1891. 

 It may be well to state that it has not as yet received very general 

 recognition in the more recent literature on the subject. It was, 

 however, fortunate enough to receive the endorsement of the late 

 Professor MILNES MARSHALL in his text-book of Vertebrate Em- 

 bryology. When the pelagic larvae of Amphioxus are confined in 

 glass jars, after a certain lapse of time they sink to the bottom, 

 like all other pelagic organisms. When they arrive at the bottom, 

 they fall over on to one side, owing to a physical impossibility to 

 rest in any other position, just as was described above for the 

 adult. It ought not to require to be emphasised that their inci- 

 dentally lying on one side is not due to a pressing desire or 



