40 K. Mi'J'ISUKUlll. 



;ibo\e, if tlic proce.-s.s liegiiii in Triuuyx i.s ("irried jn.sit une ste}» 

 litrther, the seru-ainniutic conne(.-ri<)n would ceu«e to exi.sl. Wluit i« 

 the cause which Ijrouulit about this di«a])[)earance? So far a« I can 

 «ee, the .sei'o-amniotic coiinectioii serxef- no |)ractical purpose in Ciem- 

 uiys and its presence is only to be accounted for phylogeneticaJly. 

 If such is the case, it would be undoul)tedly ecoiiomical to skip over 

 the roundabout iiiaiiner by which the aUantois spreads itselt in 

 Clem in \ s round the sero-anniiotic c(jnnecti(jn. Hence its dis- 

 appearance at last in higher forms. Whether the innnediate agent of its 

 forward shifting is the force exerted solely by the growing edge of the 

 aUantois i cannot tell. It is no doubt partly due to that, but in addi- 

 tion I oiler the following as a suggestion. In T r io n y x , the alhiiitoic 

 vessels come out synunetrically on each side; in Clemniys, the 

 symmetry is disturbed, the right set is found more anteriorly than 

 the left. As I have often remarked, CI em my s presents on the whole 

 more primitive relations, but I cannot regard this asymmetry of 

 the allantoic blood-vessels as a |)riiniti\ e (•(jndition : something being- 

 present in Clemmys has disturbed the <n-iginal symmetry and being- 

 absent in Tri onyx no longer intereferes with it and this something 

 I think is the presence of the sero-amniotic connection. May not 

 the tendency of the blood-vessels to assume a symmetrical arrange- 

 ment help to push the sero-anmiotic connection forward in Triony x? 



