434 OUTLINES OF CHORDATE DEVELOPMENT 



it disappears entirely; its proximal end may occasionally remain 

 tubular as Meckel's diverticulum of the intestine. The yolk- 

 sac remains as a very small vestige, ordinarily 5 mm. or less in 

 diameter, even up to the the time of parturition. 



4. The Allantois 



The allantois is no less variable among the Eutheria, both 

 in its mode of development and in its definitive relations, than 

 are the other embryonic appendages. And again there is little 

 parallel between allantoic characters and the usual ordinal 

 classification of the Eutheria. Among the Carnivora and the 

 lower Primates (Lemurs) the allantois becomes very large and 

 fills the exocoelomic space, while, as an opposite condition, 

 among the other Primates it forms not even a free vesicle, but 

 remains as a vestigial structure, enclosed within the umbilical 

 cord. Between these two extremes there is a great variety of 

 conditions. 



The earlier embryonic history of the allantois is considerably 

 less variable than its later history and final relations. At first 

 there is essential similarity to the condition already described 

 in the chick, so that the earlier phases in its development need 

 not be described here. Many of the later modifications of the 

 avian type of allantois are correlated with the fact that among 

 the Mammals the allantoic circulation functionally takes the 

 place of the avian yolk-sac circulation, or in other words, the 

 allantois takes an essential, though secondary, part in the 

 formation of the embryonic placental structures, indeed it may 

 even be limited to this relation, as in the rabbit. From the 

 functional relations of the allantois, among the Mammalia, it 

 is clear that its mesodermal structures, in particular its blood 

 vessels, are its most important elements, and there is relatively 

 slight variation in their arrangement. 



As in describing the amnion we may consider two of the 

 important types of allantoic formation and history, and then 

 mention briefly a few comparative points of interest. 



In the rabbit the allantois appears on the eighth day of de- 

 velopment, as a small mass of mesoderm cells extending into the 



