CHAPTER XIX 

 FOETAL MEMBRANES. 



In all Vertebrates, with the exception of Fishes and Amphibians which lay 

 their eggs in water, there begin to develop gr a very early stage certain accessory 

 or extraembryonic structures which may be conveniently called jostal mem- 

 branes. The development of these structures is very closely i elated to the de- 

 velopment of the embryo itself, and their presence is apparently largely depend- 

 ent upon the very considerable length of embryonic life in these forms, during 

 which it is necessary for the embryo to maintain a definite relation to its food 

 supply and to possess means of discharging waste products. The fcetal mem- 

 branes, therefore, have to do with the protection and nutrition of the growing 

 embryo and also are connected with the care of the waste products of fcetal 

 metabolism. 



Under the head of fcetal membranes are to be considered (i) the amnion, 

 (2) the allantois, (3) the chorion; also in connection with these, the yolk sac and 

 the umbilical cord. 



The development of these structures in Mammals and especially in man is 

 extremely complex and can be best understood by comparison with their simpler 

 development in Reptiles and Birds. 



FOETAL MEMBRANES IN BIRDS AND REPTILES. 



Throughout these two classes there is such uniformity in the formation of 

 the fcetal membranes that the chick may be taken as typical. The chief 

 characteristic of these classes, as influencing the form and structure of the fcetal 

 membranes, is the very large amount of yolk stored up within the egg for the 

 nutrition of the embryo. This is made necessary by the early separation of the 

 egg from the mother, in contrast to the close nutritional relationship between 

 mother and foetus which obtains in Mammals (excepting Monotremes), where 

 the young are retained within the body of the mother up to a comparatively late 

 developmental stage. 



The Amnion. Returning to that point in the development of the blastoderm 

 of the chick where no trace of amnion has as yet appeared, we recall that the 

 blastoderm at this stage consists of three layers, ectoderm, mesoderm and 

 entoderm; that the medial line of the embryo is marked by the neural groove, 

 flanked by the neural folds which are continuous with each other anteriorly; that 



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