According to Cliffs Bio, p. 215:
"Chorion - the outer membrane. In birds and reptiles, it acts as a membrane for gas exchange. In mammals, the chorion implants into the endometrium, later the chorion together with maternal tissue, form the placenta. The placenta is a blend of maternal and embryonic tissue across which gases, nutrients, and wastes are exchanged."
"Allantois - In birds and reptiles, it initially stores waste products. Later in development, it fuses with the chorion, and together they act as a membrane for gas exchange with blood vessels below. In mammals the allantois transports waste products to the placenta. After subsequent development, it forms the umbilical cord, transporting gases, nutrients, and waste between the embryo and the placenta."
This is strange. My initial gut feeling was allantois for mammals and chorion for birds and reptiles. I just reviewed this stuff yesterday so it is all still fresh in my mind. After RE-reading this it sounds like it should be the umbilical cord.. Is the umbilical cord considered a "membrane" in this case (I didn't think it was, thats why my gut feeling was allantois)? Also, was that the entire question? It seems like the question would specify because it is different for mammals vs. birds and reptiles. I had a couple question about this (and embryonic development in general) on some practice problems and it made me want go back to reread the embryonic development stuff to freshen up. I want to say it was q-vault and/or destroyer where I encountered these questions. Since I had my book out and a bookmark on the page I thought I would just cite what it said. Hope this helps!
So I guess technically the chorion is a wrong answer because it must become the placenta before gas exchange occurs (in mammals). In birds and reptiles it does act as a membrane for gas exchange. However, if in birds and reptiles, the allantois and chorion fuse together they act as a membrane for gas exchange together... So now were back to being the chorion or the allantois (but the chorion can do it without the allantois in birds and reptiles?)... Although in mammals, the allantois, after subsequent development, forms the umbilical cord, transporting gasses, nutrients, and wastes... This last part is what confuses me.. The umbilical cord is for the transport of gases and nutrients, so I'm guessing that the umbilical cord IS NOT considered a membrane? If anyone can clarify this it would help me, and the previous two posters I'm sure!!
*I had Cliff's open the entire time I posted this by the way, I didn't remember all these details from yesterday* I remembered Gas exchange in birds and reptiles=chorion and in mammals=allantois. But after rereading it now I'm second guessing myself because of the umbilical cord part... I'll be so happy when this DAT is over with!!!!
