THE FCETAL MRMBRANKS OF THR YFRTEr.BATES. 187 



loideau placentation precedes an allantoidean one, the allan- 

 tois being a vesicular outgrowth, as it is in so many mammals 

 and in all sauropsids. 



I cannot refrain from looking upon the vascularisation of 

 the outer larval hxyer or trophoblast, such as it occurs in man, 

 in the monkeys, and in Tarsius, as the more primitive arrange- 

 ment of the two. And in that case the presence of a connect- 

 ing stalk (Hafts ti el) and the absence of a free allantois in 

 man, monkeys, and Tarsius is not a secondary simplication, 

 but a primary fact of high importance. What is known as 

 the allantois tube inside tlie so-called Haftstiel or Bauch- 

 stiel of man, monkeys, and Tarsius, is not the remnant of 

 what was once a vesicular allantois, but a remnant of that 

 pai't of the entoderm which has served towards the vasculari- 

 sation of the trohpoblast. It is this portion of the entodermal 

 surface which will become the free allantois in those other 

 descendants of the primitive tetrapods, which have not 

 adhered to the very direct line of utilising most fully and as 

 early as possible all favourable circumstances. This most 

 direct line leads up straight to the primates. Less direct lines, 

 in which conditions of different or of slower vascularisation 

 have come to the foreground, are, however, represented in 

 various orders of monodelphian mammals, nnd further in the 

 Didelphia, the Ornithodelphia, and in the different subclasses of 

 sauropsids. In the latter the allantois has grown to the 

 dignity of a separate fcetal membrane, which co-operates to 

 the further ensheathing of the developing embryo, and which 

 carries the blood-vessels for respiratory purposes to the inner 

 surface of the egg-shell, whereas, in the ancestral viviparous 

 forms, the same vessels were more directly distributed over 

 the inner surface of the outer embryonic larval layer, in order 

 to improve the nutritory conditions which had been inaugu- 

 rated by phagocytic action of the trophoblast cells on the 

 maternal tissues. 



This, then, is a short sketch and a rapid review of how the 

 foetal membranes of the vertebrates may be looked upon if 

 we make certain changes in the interpretations that have 



