1 68 Vertebrate Embryology 



the head and neck of the embryo, while the 

 tail- and lateral-folds had started to develop, 

 but were not nearly so far advanced as the 

 head-fold. By the end of the third day, the 

 different folds of the amnion have met, and 

 covered all of the embryo except a small spot 

 at the posterior end. During the fourth day 

 the amnion becomes complete and entirely 

 covers the embryo (Figs. 37 and 57). As the 

 head-, tail-, and side-folds of the amnion meet, 

 their outer layers all fuse together to form a 

 continuous sheet, the outer or false amnion 

 (the serous membrane) ; while the inner layers 

 fuse to form the inner or true amnion (Fig. 

 38). Between the inner and outer layers of 

 the amnion is a space, continuous, as has been 

 said, with the body-cavity, into which the al- 

 lantois, presently to be described, grows. 



The space between the true amnion and the 

 embryo is the amniotic cavity ; and in it is 

 soon developed a watery fluid, which is, at 

 first, very small in quantity, but which later 

 increases enormously. 



Up to the fifth day, the amniotic cavity is 

 very small, so that the true amnion invests the 

 embryo quite closely ; but during the later 

 days of incubation, the amniotic fluid becomes 



