Chap. VI.] 



GENERAL EMBRYOLOGY. 



109 



amnion lies beneath and in close contact with the investing 

 membrane (zona radiata of the rabbit, and vitelline membrane 

 in the fowl), and is known as the serous membrane (s. m.). The 

 space beneath it may be called the subserous space (s. s. c.) \ it is 



n/m.j'^ 



FIG. 38. FCETAL MEMBRANES. 



i. Embryo folded off from yolk-sac, ii. Transverse section through embryo, 

 showing cleavage of mesoblast and amnion fold. iii. Transverse section 

 through older embryo, iv. Longitudinal section at a stage a little later 

 than iii., the amnion folds having united, v. Longitudinal section to show 

 formation of allantois. vi. Embryonic rabbit with foetal membranes. 



al. Allantois. am. Amnion. am. /. Amnion fold. &. c. Body-cavity (ccelom). 

 s. ?i. Serous membrane, s. s. c. Subserous cavity, so. I. Somatopleure. sp. I 

 Splanchnopleure. utrib. Umbilical vesicle, x. Point where serous membrane 

 joins yolk-sac, y. s. Yolk-sac. 



continuous with the body-cavity (38, iii.). Fig. 38, iv., is a 

 diagrammatic longitudinal section (the body-cavity being 

 omitted), in which the amnion folds have coalesced, the inner 

 layer forming the true amnion (am.), the false amnion forming 

 the serous membrane (s. m.). Within the inner layer or true am- 

 nion a fluid collects, in which the embryo lies as in a water-bed. 

 In the rabbit the head of the embryo is sunk into a pit or 

 depression of the blastodermic vesicle, from the edge of which 

 the amnion fold takes its origin. In this depression, termed the 

 proamnion, the mesoblast is absent. It is shown in 38A, iii. The 



