908 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG 



tissues, together with a peristaltic behavior of the oviducal musculature, affords 

 the mechanism necessary to transport the eggs to the external environment. 

 Egg membranes are not deposited around the egg as it passes through the 

 oviduct in teleost fishes. 



In the elasmobranch fishes, however, glandular and uterine portions of the 

 oviduct are present, and the large egg is transported through the upper 

 glandular region of the oviduct in a manner similar, presumably, to that in 

 the hen. Surrounding membranes of albuminous materials, and an outer chiti- 

 noid "shell" are produced in the glandular area. These membranes vary with 

 the species and some are complicated as indicated in figure 380A. In many 

 elasmobranch fishes and also in the so-called viviparous teleost fishes, the 

 egg is retained in the uterine portion of the oviduct. Here the young develop 

 and, when discharged to the outside, are able to fend for themselves. In these 

 forms, the uterus is adapted to the function of providing the embryo with 

 an environment suitable for its development. 



In the cyclostomatous fishes, an oviduct is not present, and egg transport 

 resolves itself into a discharge of eggs into the coelomic cavity from which 

 the eggs pass through openings into the cavity of the urogenital sinus. Ovarian 

 membranes only are present around the cyclostome egg. These membranes 

 may be complex as in the hagfish, Polistotrema (Bdellostoma), (fig. 162). 



2. The Extra-embryonic Membranes 



The extra-embryonic membranes as indicated previously are those mem- 

 branes produced from the embryonic tissues. These membranes are the yolk 

 sac, amnion, chorion (serosa) and allantois. In a strict sense, the periderm 

 (see Chapter 12) probably should be included as an extra-embryonic mem- 

 brane for it is elaborated at the surface area of the epidermis and functions 

 to protect and presumably to regulate the possible entrance of substances 

 from the surrounding environment. 



a. Yolk Sac 



A yolk sac is present in all reptiles, in birds and mammals, and in those 

 fishes which have megalecithal eggs, that is, having a large amount of yolk 

 substance stored within the egg. Two types of yolk sacs are found among 

 the vertebrates, viz.: 



( 1 ) a yolk sac whose walls are composed of entoderm, mesoderm and 

 ectoderm in the form of closely associated layers. This type of yolk 

 sac is found in the embryos of the hagfishes, Polistotrema (Bdellostoma) 

 stouti and Myxine glutinosa, in most elasmobranch fishes, and in 

 teleosts (fig. 370A). Some of the amphibia with a large quantity of yolk 

 in the egg such as Necturus maculosus, also approach this condition. 



(2) a second type of yolk sac is found in reptiles, birds and mammals. In 

 these instances the wall of the yolk sac is composed mainly of ento- 



