310 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



into depressions in a corresponding area of the uterine mucous 

 membrane, which becomes thickened and vascular. A blood- 

 circulation is developed in the allantois, and blood-vessels extend 

 into the villi. Ultimately, close union takes place between the 

 villi and the wall of the uterus. A circular flattened structure, 

 the placenta, is thus formed, consisting of an embryonic part, 

 mainly formed by the allantois, and a maternal part, constituted 

 by the thickened area of the uterine mucous membrane. The 

 placenta is connected with the embryo by the stalk of the 

 allantois, and certain structures which surround and form with 

 it the umbilical cord. The impure blood of the embryo passes 

 into the embryonic part of the placenta by two allantoic arteries 

 which break up into branches, and, in the villi, come into close 

 relation with the blood-vessels of the mother. There is not, 

 however, an actual union. Waste-products thus pass from the 

 blood of the embryo into the blood of the mother, which, on the 

 other hand, supplies nutritive material and oxygen. The lack of 

 food-yolk is thus compensated for. (For details of the embryonic 

 circulation, see p. 267.) 



Several embryos develop in each uterus at the same time, 

 occupying separate swellings. At birth the embryonic mem- 

 branes are cast off, including the embryonic part of the 

 placenta. 



CHAPTER XIIL COMPARATIVE ANIMAL 

 MORPHOLOGY AND PHYSIOLOGY. 



THE comparison of animals is perhaps best conducted on a 

 physiological basis, taking the headings of nutrition, katabolism, 

 reproduction, contractility, irritability, and spontaneity (see 

 Amoeba) and adding to these the subordinate one of protection 

 and support. 



Amoeba, Vorticella, and Gregarina, unicellular animals, perform 

 all the functions of life, the first being but little differentiated, 

 the second a great deal, the third modified as a result of para- 

 sitism. From this point onwards the multicellular condition leads 



