v.] THE AMNION. 107 



and the duct itself also passes inwards relatively to the 

 epiblast (fig. 43 wd). Its hind end elongates till it 

 comes into connection with, and opens on the fourth 

 day into the cloacal section of the hind-gut. 



The amnion and allantois. The amnion, especially 

 the anterior or head fold, advances in growth very 

 rapidly during the second day, and at the close of the 

 day completely covers the head and neck of the embryo ; 

 so much so that it is necessary to tear or remove it when 

 the head has to be examined in hardened opaque speci- 

 mens. The tail and lateral folds of the amnion, though 

 still progressing, lag considerably behind the head-fold. 



The side-folds eventually meet in the median dorsal 

 Una, and their coalescence proceeds backwards from the 

 head-fold in a linear direction, till there is only a small 

 opening left over the tail of the embryo. This finally 

 becomes closed early on the third day. 



In Figs. 32 and 43 am. the folds of the amnion are 

 shewn before they have coalesced. After the coalescence 

 of the folds of the amnion above the embryo the two 

 Umbs of which each is formed become, as already ex- 

 plained in chapter II., separate from each other: the 

 inner, forming a special investment of the embryo, and 

 constituting the amnion proper (Fig. 65), the outer at- 

 taching itself to the vitelline membrane and becoming 

 the serous envelope. 



The development of the allantois commences during 

 the second day, but since it is mainly completed during 

 the third day we need not dwell upon it further in this 

 place. 



Summary. The chief events, then, which occur 

 during the second half of the second day are as follow: — 



