484 ON THE ZOOLOGICAL POSITION 



Patterson, that a genuine connecting stalk, presenting a re- 

 markable similarity to that of Tarsius, is also present in the 

 Armadillo {2'atusia novemcinctci), in which obviously it must have 

 been evolved quite independently of that of the Primates. 

 Nevertheless, the common occurrence of this structure in these 

 two groups of Primates is a feature of very great interest, and 

 the first question we have to consider is its functional and 

 morphological significance. When fully established, it consists 

 of a strand of mesoderm, into which there extends a tubular 

 diverticulum (known as the allantoic duct) from the hind-gut of 

 the embryo, and it serves to connect the posterior end of the latter 

 directly with the chorion (text-figs. 4 & 5, est.). It is simply 

 a mesodermal short-cut between the embryo and the enclosing 

 chorion, and its function is to facilitate the early vascularisation 

 of that membrane by furnishing a direct path for the umbilical 

 (allantoic) vessels which foi'm the essential fcetal constituent of 

 the definitive or allantoic placenta. In the lower Mammals the 

 chorion is vascularised as the result of the secondary union with 

 it of the vesicular allantois, which likewise carries the umbilical 

 vessels. Prior, however, to the establishment of the functional 

 allantoic placenta, the nutrition and respiration of the embryo in 

 the lower Mammals are provided for by means of a temporaiy 

 omphalopleural or so-called yolk-sac placenta, involving the 

 vitelline or yolk-sac vessels of the omphalopleure or primitive 

 blastocyst- wall. 



Now, in the Lemuroids, as we have seen, the allantois xmites 

 with the chorion relatively' early, and the entire omphalo- 

 pleure is rapidly resolved, through the extension of the extra- 

 embryonal coelom, into chorion and yolk-sac w^all, with the 

 result that a yolk-sac placenta, if it exists at all, is of quite 

 transitory duration. In I'arsius and the Anthropoids, owing to 

 the much earlier differentiation of the entodermal yolk-sac and 

 the chorion, a yolk-sac placenta cannot be formed at all, and so as 

 a compensation what appears to have happened in these forms is 

 that the allantois, and more particularly the vessel-carrying 

 allantoic mesoderm, became precociously developed in the form 

 of a solid cord, running directly from the hinder end of the 

 embryo to the attached area of chorion, marking the site of the 

 future placenta. 



The entodermal lining of the allantois at the same time under- 

 went reduction, and now appears in the form of a diverticulum, 

 usually slender and tubular, which runs from the hind-gut for a 

 longer or shorter distance into the mesoderm of the cord. 



This entire structure, then, is the connecting stalk, and what 

 I want to insist on is that it is not something new nor is it a 

 primitive formation (as Hubrecht maintained). It is none other 

 than a precociously formed and adaptively specialised allantois, 

 the object of which is to provide for the early and direct vascu- 

 larisation of the chorion — or, in other words, for the nutrition and 

 respiration of the embryo at the earliest possible moment. That 



