Foetal Mernbraues of the American Beaver (Castor cauadensis). 



209 



•'downwards" with relation to the placenta) to the side of the chorion 

 as already described, adhering to the amnion until it reaches the 

 chorion. The allantoic vessels pass backwards to be distributed 

 throughout the spongy placenta. At the point where the omphalo- 

 mesenteric and allantoic vessels diverge from the common umbilical 

 cord, they are suspended to the foetal surface of the placenta by a 

 ligament, like a mesentery, formed by apposition of sheets of the 

 amnion; this may be named the omphalo-mesenteric liga- 

 ment (Fig. H — K). It acts as a suspensory ligament for the umbilical 

 cord which maintains a horizontal course, parallel with the ventral 

 surface of the foetus, until it abuts upon the placenta. 



The embryo itself 

 is not suspended by 

 its umbilical cord, but 

 by the insertion of 

 the amnion upon the 

 surface of the placenta. 

 This suspensory func- 

 tion of the mammalian 

 amnion is one which, 

 I believe, is not com- 

 monly emphasized. 

 When a specimen, 

 prepared as shown in 

 Fig. J, is held up by 

 grasping the placenta 

 in the hand. the weight 

 of the embryo is 

 entirely sustained by 

 the amnion, the um- 

 bilical cord remaining 

 horizontal. 



In another spe- 

 cimen. the umbilical 

 cord was much more 

 convoluted , its hori- 

 zontal course short, 

 and it reached the pla- 

 centa near its caudad 

 end ; the omphalo- 



Fig. K. 



Foetal beaver removed from its amniou, showing the 



attachment of the umbilical cord and allantoic vesicle 



to the placenta. 



1. Umbilical cord. 2. Omphalo-mesenteric cord. 



3. Omphalo-mesenteric ligament. 4. Placenta. 5. Amnio- 



placental insertion. 6. Allantoic vesicle. 



14 



Zool. Jahrb., Suppl. XV (Festschrift für J. W. Spengel Bd. II). 



