CHAPTER XXIII 

 PARENTAL AND EMBRYONIC NOURISHING TISSUES 



MANY organisms cast their ova out into the surrounding water, to 

 develop and perpetuate the race by sheer chance and force of numbers. 

 Others place them in "nests," and some of these even go about with the 

 young for some time afterward, aiding them by their protection from 

 enemies and in their search for food. The parent may even bring them 

 food until they can procure it themselves. Still another cjass of parents 

 feed the young both before and after birth with some product or part of 

 their own body. It is with the tissues whereby they perform this latter 

 function, that this chapter deals. 



The most frequent method of supporting the young is inside the 

 body, in contact with some surface through which the food can be passed 

 and the waste products returned to the parent's blood. This process 

 probably had its origin in many of the organisms which kept the young 

 in the body for development on their egg-yolk supply. Such a method 

 may be observed in hundreds of forms, some of them very lowly organized. 

 Sometimes the attachment to the parent's body is maintained through 

 the membranes of the mother's or father's mouth, as in a catfish, As- 

 predo batrachus, or the skin of the back or belly as in a toad, Pipa 

 Americana. This external connection possibly furnishes the young with 

 some benefits in its growth, possibly some small amount of fluid food. 



The spiny dogfish, Acanthias vulgaris, is one of the animals which 

 retain the ova in the reproductive passages during development. The 

 egg is very large, and when discharged from the ovary it passes into the 

 oviduct, and in the upper part of this tube it is provided with a watery 

 jelly layer and, later, with an outer covering of thin, transparent, flexible 

 material which easily ruptures, but which, in other sharks and rays, is 

 tough and heavy. 



When the egg is developed so that the young animal is of some size, 

 this outer shell breaks, and the young animal lies in the dilated lower 

 portion of the oviduct, which thus becomes the uterus. Here it lies in a 

 fluid, and the walls of the uterus become specialized to present a larger 

 blood supply and a greater surface for exchanges between the blood 

 and the uterine fluid. 



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