THE FCETAL MEMBRANES OF THE VEETEBEATES. 181 



trophoblast g'oing parallel with ifc, a yet more efficient mode of 

 nutrition than the one alluded to above under (3) might be 

 obtained if the embryonic vascular system, which was slowly 

 coming- into existence on the hereditary plan of development, 

 succeeded in spreading- out, in one way or another, on this 

 outer trophoblastic layer, and would enter into osmotic inter- 

 change with maternal blood. 



Finally, the protection of the embryonic shield during its 

 further development by some sort of appliance resembling a 

 water cushion would, in these incipient viviparous animals, 

 undoubtedly have been a most efficient variation, for the 

 earliest origin of which we have simply to go back to the 

 early stage in which we noticed the formative cells of the 

 embryo adhering to the larval layer, the trophoblast, in one 

 spot only. Suppose that in further development this sessile 

 attachment to have become converted into a circular adhesion 

 — by fluid accumulating between the trophoblast cells and the 

 formative cells, as we see it happen under our eyes in Erinaceus 

 and Gymnura — we then find that the water-cushion, in casu 

 the amnion, took its origin in a most simple fashion, whereas 

 the chorion is in no way dependent on it, but has preceded it 

 as as earlier formation. 



The rapid summary here given shows us that the assumption 

 of a single monodermic larval layer is quite far-reaching 

 enough to allow us to understand how, out of it, chorion, 

 amnion, and allantois (the latter as representing one form of 

 early vascularisation of the trophoblast) have gradually come 

 about. 



The only change we have to make, in what I might 

 designate the present " fashion" in comparative embryology, 

 is that we look upon the earliest ancestors of mammals not as 

 oviparous, yolk-laden vertebrates, but that we acknowledge 

 them to have been viviparous animals with blastocysts that 

 obtained vesicular shape from quite other motives than an 

 eventual " loss of yolk," such as Rabl has attempted to prove. 

 Here, then, is the place for an appeal to paleeontologists. They 

 have no shadow of direct interest in foetal envelopes which are 



