INDEX 573 



X yolk stalk, 69, 70 



Xenarthra, 37. Z 



Y zona radiata, 52 



zonary placenta, 72 



yolk, 52, 53, 69, 73 zygapophyses, 508 



yolk sac, 69, 70, 319 zygote, 7, 48, 58. 





