254 COMPARATIVE ZOOLOGY OF CHORDATES 



In the other type, of which the mouse is an example, the 

 amnion arises as a cavity hollowed out in the inner mass of cells, 

 within the trophoblast. This method is called amnion- 

 formation with entypy of the germ. In this case, there are 

 no amniotic folds, and the trophoblast (which forms a complete 

 investment, as in the rabbit) does not become interrupted by 

 any disappearance of Rauber's cells. When the amniotic 

 cavity is formed in the mouse, the embryo becomes differ- 

 entiated on its floor. The mouse therefore starts from a 

 condition which the reptiles do not reach until a fairly late 

 stage of development, when the amniotic folds have been 

 formed. 



The amniotic cavity contains fluid, and this enables the 

 embryo to develop in a fluid medium, although its egg was not 

 laid in water. 



It is interesting to note how in the higher vertebrates 

 certain processes take place as if they were abbreviations of the 

 conditions which prevail in lower vertebrates. To start with, 

 the primitive streak which is the beginning in higher verte- 

 brates, represents a stage which the lower vertebrates only 

 reach after the blastopore has formed, and become closed by 

 the apposition of its lateral lips. Similarly, the mammal with 

 a hollowed-out amniotic cavity within the trophoblast starts 

 from a condition which the reptiles and birds reach after the 

 upgrowth and fusion of the amniotic folds. Not only this, 

 but in such mammals the amniotic cavity arises first and the 

 embryo forms in its floor ; whereas in the reptiles and birds the 

 embryo forms first and the amniotic folds arise afterwards. 

 This process of " short-circuiting " and telescoping of develop- 

 mental processes reaches its climax in the Primates, where the 

 body-stalk develops first and the allantois grows into it later. 

 In lower mammals as well as in reptiles and birds, the allantois 

 grows out from the hind gut at a fairly late stage. In the 

 Primates, the conditions are as if everything were first got 

 ready for the embryo, after which it makes its appearance. 

 This is not without interest in connexion with the superior 

 organisation and differentiation of the highest mammals, for 

 this superiority in construction is dependent on a prolonged 



