THE EMBRYOLOGY OF THE OPOSSUM 99 



swim bladder of fishes; so the long accepted theory of the 

 homology of the lungs and the swim bladder is not contra- 

 dicted by this interpretation they are homologous, and they 

 are both gill pouches. 



The tail fold of the amnion. The anterior and the posterior 

 amniotic folds in the opossum are of different constitution 

 and never completely merge. The definitive amnion thus has 

 two quite distinct portions, which Selenka (loc. cit.) recognized 

 and accurately described. 



After pointing out that the pro-amnion, which van Beneden 

 and Julin first described in the rabbit and the bat, is a transi- 

 tory structure which contributes nothing to the definitive 

 amnion in all other known cases, he says: 



Beim Opossum umhiillt das aus Ekto- und Entoderm beste- 

 hende Kopf amnion (wie ich das von Van Beneden und Julin 

 als 'Proamnion' beziechriete Gebilde nennen will) vier Tage 

 nach Beginn der Furchung [stage 27] ungefahr des vordere 

 Drittel des Embryonalkorpers, wahrend das Rumpfamnion, 

 welches aus Ekto- und Mesoderm zusammengesetzt ist, dessen 

 hintere zwei Drittel umfasst. Am Ende des fiinften Tages 

 [stage 29] sind beide Falten fast gleich gross und am Ende 

 des sechsten Tages [stage 32] ist der ganze Embryo aus- 

 schliesslich vom Kopf amnion (Ekto- und Entoderm) um- 

 kleidet, wahrend das Rumpfamnion sich hinter den Schwanz 

 zuriickgezogen hat. Das Kopfamnion spielt also hier die 

 Rolle eines Dauerorgans, hingegen ist das Rumpfamnion das 

 transitorische Gebilde geworden. 



The closure of the amniopore is illustrated in figure 34. 

 The germ layer constitution of the amnion can be seen in the 

 reconstruction of stage 29 (fig. 39). The final proportions 

 of the head-amnion and trunk-amnion referred to by Selenka 

 are shown in stage 33 (fig. 52). 



Abnormalities are fairly common among opossum embryos. 

 I have one specimen of stage 29 which has a head fold back to 

 the limb buds as in stage 28, figure 34, but no tail fold at all. 

 Another specimen has a tail fold like that of a normal stage 27, 

 but no head fold at all. That these specimens represent real 

 abnormalities, not merely extremes of normal variation, is 



