418 OUTLINES OF CHORDATE DEVELOPMENT 



wall is the embryonic respiratory organ, while its cavity serves 

 as an excretory reservoir; and the chorion (serosa) appears to 

 have little, if any, physiological importance. In the Mammal 

 this is all changed. The amnion is a membrane of secondary 

 protective importance; the yolkless yolk-sac is a vestigial organ, 

 often of little functional value; the allantois loses its respiratory 

 and excretory significance and is usually concerned in relating 

 the embryo to the source of its food supply; while the chorion, 

 either as a whole or in part, becomes the chief organ concerned 

 in the exchange of nutritive materials and excreted substances 

 between the embryo and the maternal uterine circulation. 



These characteristic relations of the mammalian embryonic 

 membranes do not appear in this group in a fully established 

 condition; there is, on the contrary, a long series of intermediate 

 conditions, transitional in almost every respect, between the 

 Sauropsid condition and that found in the highest Mammals, 

 the Primates, where these relations are most highly developed. 

 It is a familiar fact that in the lowest Mammals, the Prototheria 

 (Monotremata or Ornithodelphia) including only the genera 

 Ornithorhynchus, Echidna, and Proechidna, essentially Sauropsid 

 conditions obtain here, as well as in so many morphological 

 and physiological characteristics of the adults. Here the de- 

 veloping embryo has no organic relation with the mother, for 

 the fully formed eggs are deposited outside the body of the 

 parent, either in an integumentary fold (Echidna), or in a 

 "nest" (Ornithorhynchus), where the young develop indepen- 

 dently, enclosed within a tough parchment-like egg shell. Usu- 

 ally only a single egg is produced at one time in Echidna, while 

 Ornithorhynchus normally produces two at a time. The eggs 

 too are reptilian in character, much larger than any other 

 mammalian egg, and yolk laden. As laid, they measure about 

 15-16.5X12-13 mm. (in Echidna); the egg cell proper, as it 

 leaves the ovary is, of course, smaller than this, but even so 

 is much larger than the egg cell of the higher Mammals, being 

 3.0-4.0 mm. in diameter in Echidna, 2.5 mm. in Ornithorhynchus. 



Among the Metatheria (Didelphia or Marsupialia) many of 

 the typical mammalian conditions are found. The ova, though 



