1606 
superseded afterwards by ingrowing mesoderm. SereNKA explains 
this phenomenon in a subtle manner by supposing the proamnion 
to have originally possessed mesoderm and bloodvessels. By the 
folding process the respiratory function of this part of the area 
vasculosa is in danger of being injured and therefore the mesoderm 
retires’). We shall revert to the subject later on, but the mere 
fact that in many Anamnia a proamnion-area is present in front 
of the enteron, without any folding process taking place, makes this 
explanation rather improbable. 
According to the opinion of SELENKA the outgrowth of the tail- 
fold is caused by the powerful growth of the allantois. The suppression 
of the free larval period and the development of the embryo within 
the eggshell render the emptying of the excretory products into 
the eggehamber impossible without at least serious injury to the 
normal metabolism of the embryo. Therefore the embryonic urine- 
bladder (allantois) expands and pushes between somato- and splanchno- 
pleura of the yolksack. By this circumstance the respiratory function 
of the yolksack, already diminished by the liquefaction and shrinking 
of this organ, will be Jargely impaired. The allantois, if sufficiently 
provided with bloodvessels, can take over this function and the 
outgrowth of the tailfold is only a means for surface-increasing in 
connection with this new function. 
Against both views serious objections may be raised. Without 
laying stress upon the rough mechanical conception of the sinking- 
down-hypothesis, we can state the fact that, along with the formation 
of the proamnion, no real sinking-in of the headregion occurs, but 
that principally the ectoderm and in second instance the entoderm 
actively grow over the embryo. Besides I never have found it 
mentioned that the proamnion fold contains yolkmaterial, a circum- 
stance that seems to me a conditio sine qua non for the sinking-down- 
hy pothesis. 
The explanation of the outgrowth of the tailfold by the supposed 
tendency to surface-increasing of the allantois is eo ipse more 
plausible, but ontogenetic facts do not agree with it. The tailfoid does 
not arise as an outgrowth of the allantois, but develops independently 
of this organ, and consists originally of ectoderm and mesoderm 
only, the true entodermal allantois following long afterwards, usually 
preceded by a so-called mesodermal one. Moreover the fact that the 
tailfold is wanting in many Sauropsida with a primitive type of 
development (Sphenodon, Chelonia) makes this explanation rather 
improbable. 
EY SELENKA, l.c. 1892, p. 187. 
