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YVETTE W. KUNZ 



of the dorsal ectoderm, which have grown downwards between eye cup and yolk 

 periblast. The formation of the headfold, however, is not described (Fig. 2a-b).l 



Also Lebistes reticulatus has the head buried in the yolk. Here, however, 

 the headfold grows downward at the 3 somite stage, before the eye cups are 

 formed. The development of the lens takes place in the usual way. This very 

 early formation of the ectodermal headfolds is a prerequisite for the formation 

 of the pericardial folds (Fig. 2e, f). 



In order to compare the appearance of the ectodermal headfold in thèse 

 three species, one looks for an "independent" developmental phenomenon. 

 The relative embryonic âge cannot be used, since teleosts hatch at widely différent 

 stages of development (Kunz, 1964). If the number of somites, related to the 

 full number, is taken, the line Lebistes -> Trout -> Serranus clearly émerges, 

 Lebistes reticulatus showing the earliest appearance. The optic primordia, 

 forming immediately after the neural cord has been laid down, constitute another 

 valid stage for comparison, and confirm the séquence quoted above. The onset 

 of pigmentation in the retina, another useful stage for comparison (Kunz, 1964), 

 cannot be employed here, since data on Serranus atrarius are lacking. 



2) Comparison of pericardium in cyprinodont families 



The Cyprinodontes comprise seven families; the condition of the pericardium 

 has been studied in five. 



A) Cyprinodontidae (oviparous) 



Ryder (1885) was the first to observe a "vascular membrane on either side 

 of the head" in Fundulus. Turner (1940b) reported that the vascularized walls 

 of the pericardium of Fundulus heteroclitus enclose the anterior part of the head 

 and extend laterally up to the middle area of the eyes. A similar description 

 was given for the pericardium of Oryzias by Kamito (1928) and by Rugh (1941). 



B) Poeciliidae (viviparous) 



Ryder (1885) observed in Gambusia patruelis a "tubular process of the yolk 

 bag which is prolonged upwards over the head behind the eye to meet its fellow 

 of the opposite side". Molliensia latipinna, Xiphophorus helleri and Lebistes 

 reticulatus are described by Turner (1940b) as having a less expanded pericardium 

 than Fundulus heteroclitus. The allegedly smaller pericardium in thèse cases is 

 said, by Turner, to be compensated for by the extension of the yolk sac in the 

 form of a broad neckstrap. Purser (1938) described a neckstrap in Lebistes 

 reticulatus; she did not, however, specify its histological nature. It is obvious 



