FORMATION OF PROTECTIVE EMBRYONIC MEMBRANES 903 



5. Mechanisms for Oxygen Supply and Carbon Dioxide Removal 



Two types of oxygen supply and carbon dioxide removal mechanisms are 

 encountered. In the majority of fishes and in the larger number of Amphibia, 

 the surface of the developing egg functions as a respiratory membrane. In 

 some fishes, and in rare instances in the Amphibia, special diverticula of the 

 pharyngeal area are developed to care for this function. On the other hand, 

 in all reptiles, birds, and mammals, the allantoic diverticulum from the hind- 

 gut assumes respiratory responsibilities. 



6. Oviparity, Ovoviviparity, and Viviparity 



The word oviparous is derived from two Latin words, namely, ovum, egg, 

 and parere, to bring forth. Oviparous animals thus produce eggs from which 

 the young are hatched after the egg is laid or spawned. Among the vertebrates, 

 oviparous species include most of the fishes, amphibia, reptiles, birds, and 

 prototherian mammals. Ovoviviparity is a condition in which the egg is re- 

 tained within the confines of the reproductive duct or other specialized areas 

 where it hatches, and the young are brought forth or born alive. The greater 

 portion of the embryo's nourishment is derived from the nutritive materials 

 within the egg, while oxygen uptake, together with fluid substances and the 

 elimination of carbon dioxide, is effected through the oviducal wall and its 

 blood vessels. Ovoviviparous species include certain sharks, teleosts, certain 

 urodele and anuran amphibia, and various reptiles. In viviparity (Latin, vivus, 

 alive) the new individual is brought forth alive. In viviparity the developing 

 embryo obtains some or all of its nourishment through the wall of the uterus 

 or other specialized structure. Viviparous forms are found among the sharks, 

 teleosts, and reptiles, together with all species of metatherian and eutherian 

 mammals. 



B. Foimation and Importance of the Protective Embryonic Membranes 



1. The Egg Membranes 

 a. Primary and Secondary Egg Membranes 



The formation of the primary and secondary egg membranes were described 

 in Chapter 3. The importance of these membranes formed around the egg, 

 while it develops in the ovary, is considerable. The so-called fertilization mem- 

 brane, produced, for example, in Amphioxus, the zona radiata and chorion 

 of fishes, the vitelline membrane of amphibians, reptiles, and birds, or the 

 zona pellucida of mammals are important structures. All these membranes 

 form the first or primary protective coating around the embryo. Between the 

 embryo and this primary embryonic membrane is a fluid-filled area, the 

 perivitelline space. The perivitelline fluid is favorable to the embryo. Thus, 

 the surrounding fertilization, vitelline, or zona membranes act as an insu- 

 lating wall between the outside environment and this early perivitelline pond 



