902 CARE ANn NOURISHMENT OF THE DEVELOPING YOUNG 



adapted to fulfill certain definite functions necessary to the embryo. The 

 extra-embryonic membranes are: 



( 1 ) The yolk sac, found in most species. The yolk sac is developed as an 

 extension of the primitive gut. 



(2) The amnion, representing a sac-like structure which surrounds the 

 embryo. It is found only in the Amniota, that is, the reptiles, birds, 

 and mammals. 



(3) The allantois. This structure arises as an outpushing from the mid- 

 ventral area of the hindgut, and is found only in reptiles, birds, and 

 mammals. 



(4) Pharyngeal diverticula. The pharyngeal diverticula are found in cer- 

 tain species of fish and in amphibians. The external gill filaments of 

 the shark embryo mentioned in Chapter 14 are an example of this 

 type of extra-embryonic membrane. Also in certain species of Am- 

 phibia elaborate pharyngeal placentae are evolved which function in 

 a respiratory capacity. 



The uterine portion of the oviduct functions, of course, as a capsule to 

 protect the developing egg in all ovoviviparous and viviparous species. 



Uncommon, specialized structures for the protection of the developing 

 embryo are formed in many species of fishes and Amphibia. These structures 

 are described more explicitly on p. 915. 



4. Types of Food Sources 



There are two main types of food sources for vertebrate embryos, namely, 

 endogenous and exogenous sources. The endogenous form of food supply is 

 found in all amphibian species, in the lung-fishes, Amphioxus, etc., where 

 nourishment necessary for development is incorporated directly within the 

 developing embryonic cells from the beginning cleavages of the egg. On the 

 other hand, in the exogenous type of food supply the nourishment necessary 

 for development lies outside of the developing embryonic tissues. This type 

 of food storage is found in elasmobranch and teleost fishes, reptiles, birds, 

 and mammals. Two categories are to be observed, as follows: 



( 1 ) In the majority of fishes, and in all reptiles, birds, and prototherian 

 mammals, the food is stored within the egg. The developing embryo 

 which lies upon this food source utilizes a specialized type of extra- 

 embryonic tissue to digest and assimilate the food materials. 



(2) In some fishes and in the metatherian and eutherian mammals, most, 

 or practically all, of the food elements come directly from the maternal 

 (and, in some instances in fishes, from paternal) tissues as the embryo 

 develops. Here also, a specialization of extra-embryonic tissue is nec- 

 essary to tap the supply of food. 



