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YVETTE W. KUNZ 



fold being formed. The Goodeidae and Yenynsiidae are "outsiders", since their 

 gestation is not entirely follicular; during the early stages in the follicle they show 

 a hypertrophie pericardium, while later they develop spécial structures which are 

 looked on as substitutes for the receding pericardial sac. 



3) HOMOLOGY OF ECTODERMAL AND PERICARDIAL HEADFOLDS OF POECILIIDS 

 WITH AMNIOTIC FOLDS OF HIGHER VERTEBRATES 



Various authors have suggested this homology (Fraser, 1940; Turner, 

 1940a-e and Tavolga, 1949). Terms such as "pseudoamnion", "pericardial 

 amnion" and "pseudochorion", "pericardial serosa" have been coined to dénote 

 the walls of the headfolds in cyprinodonts. In the following the expressions 

 "pericardial amnion" for the inner wall and, since it is vascularized, "pericardial 

 chorion" for the outer wall are used. 



A ) Morphology 



There are several basic différences between the folds of Lebistes reticulatus 

 and the amniotic folds of higher vertebrates: 



a) In ail poeciliids the pericardial amnion lies close to the head ectoderm; there 

 is never, in its development or régression, an amniotic cavity présent. 



b) The amniote embryo becomes entirely covered by the amnion. In the 

 cyprinodonts even with the most extensive overgrowth of headfolds, the caudal 

 région remains free. 



c) In the amniote, headfold and latéral folds eventually meet with the tail fold, 

 to form one continuous extra-embryonic coelom. This is not so in Lebistes 

 reticulatus where a chorio-amniotic connection (somatopleural bridge) persists 

 along the mid-dorsal line. No other author, as far as can be ascertained, has 

 described the genesis of the poeciliid headfolds, but Fraser (1940) describes, 

 for the fully developed sac of Heterandria formosa, two mesothelial strands 

 along the midline and overlaid by a continuous ectoderm. The extra- 

 embryonic pericardial cavity of Platypoecilus maculatus, however, is shown 

 to be dorsally continuous (Tavolga and Rugh, 1947). Thus, no other species 

 seems to possess the 4-layered chorio-amniotic connection, observed in 

 Lebistes reticulatus. 



B) Formation 



In the sauropsida the origin of the amnion is by amniotic folds (pleuramnion). 

 A similar mode is adopted by certain mammals; in others the amniotic cavity is 

 the resuit of delamination (schizamnion) (Portmann, 1965) (Fig. 8A-B). 



