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YVETTE W. KUNZ 



The pericardial amnion of Lebistes reticulatus is the resuit of a folding process 

 (Fig. 8C). Because of the absence of an "amniotic cavity", the folding process 

 is difficult to follow. The ectodermal headfolds proceed first in a ventral direction 

 until they meet (Fig. 2f, 7a). Later, the folding process continues in dorsal 

 direction, starting at the base of the "ventral" headfolds just formed (Fig. 7b-c). 

 The formation of thèse "dorsal" headfolds seems to be caused by the dorsal 

 expansion of the pericardium (pericardial headfolds). Figure 8C (right) showsi 

 the formation of the "ventral" and "dorsal" part of the ectodermal headfolds 

 superimposed, and also the formation of the pericardial headfolds. After 

 completion of the folding processes, the layer that originally constituted the inner 

 sheet of the ectodermal headfold is now the head ectoderm, while the outer sheet 

 forms part of the "amnion". 



4) Analogy of ectodermal and pericardial headfolds 

 with amniotic folds 



The phylogenetic origin of the amniotic folds is obscure. Since it is accepted 

 that reptilia and birds were the first to lay eggs on dry land, it is suggested thaï 

 the amnion has been developed to keep the embryo moist. In the placenta 

 mammal desiccation présents no problem, and the function of the amniotic 

 cavity here is thought to be one of protection, i.e. equalizing and distributing 

 pressure exerted against the embryo. 



Since the poeciliids are devoid of the amniotic cavity, the extra-embryonic 

 pericardial cavity enclosing the head may be assumed to act as a pressure cushior 

 (40 embryos in one ovary, on average). It may be suggested, however, that the 

 main significance of the "amniotic folds" in Lebistes lies in the vascularized outei 

 wall or pericardial chorion. This is an extension of the vitelline circulation, and 

 its functions must, therefore, be additive. The functions of the teleostean vitelline 

 circulation are thought to be similar to those of the chorion of the amniota 

 namely nutrition, respiration and excrétion. One should therefore, with this ir 

 view, limit the functions of the pericardial chorion to the following: 



A ) Accessory respiratory function 



Respiratory function of the expanded pericardial sac was put forward b) 

 Ta volga and Rugh (1947) for Platypoecilus maculatus. There seems to be some, 

 indirect évidence for this function in Lebistes reticulatus because the vascularizatior 

 of the pericardial sac is at its maximum (15th embryonic day) well before respira- 

 tory exchange is taken over by the gills. By the time the headfolds have béer 

 reduced to the neckstrap (20th embryonic day), the pectoral fins beat continuously 

 thereby producing a vigorous circulation of the perivitelline fluid, which benefitJ 



