A VEX. 139 



yolk-sack. The general nature of the process is seen in the diagrams 

 tigs. lOo and 121. The folds by which it is effected are usually dis- 

 tinguished as the head-, the tail- and the lateral folds. The head-fold 

 (tig. 105) is the first to appear ; and in combination with the lateral 

 folds gives rise to the anterior part of the mesenteron (D) (including 

 the oesophagus, stomach and duodenum), which by its mode of forma- 

 tion clearly ends blindly in front. The tail-fold, in combination with 

 the two lateral folds, gives rise to the hinder part of the alimentary 

 tract, including the cloaca, which is a true part of the mesenteron. At 

 the junction between the two folds there is present a circular opening 

 leading into the yolk-sack, which becomes gradually narrowed as 

 development proceeds. The opening is completely closed long before 

 the embryo is hatched. Certain peculiarities in reference to the struc- 

 ture of the tail-fold are caused by the formation of the allantois, and 

 are described with the embryonic appendages. The stomodavum and 



Fit;. Id."). DIAGRAMMATIC LONUITUMNAL SECTION THKHCI.H THE AXIS OF AN EMBRYO BIRD. 



The section is supposed to be made at a time when the head-fold has commenced 

 but the tail-Cold has not yet appeared. 



F.So. head-fold of the somatopleure. F.Sjt. head-fold of the splanchnopleure. 



pp. pleuroperitoneal cavity; Am. commencing (head-) fold of the amnion ; D. ali- 

 mentary tract ; X.C'. neural canal ; Cli. notochord A. epiblast ; B. mesoblast ; C. hy- 

 poblast. 



proctodseutn are formed by epiblastic in vagi nations. The commu- 

 nication, between the stomodseum and the mesenteron is effected 

 comparatively early (on the 4th day in the chick), while that between 

 the proctodseum and mesenteron does not take place till very late 

 (loth day in the chick). The procfodaeum gives rise to the bursa 

 Fabricii, as well as to the anus. Although the opening of the anus is 

 so late in being formed, the proetodseum itself is very early apparent. 

 Soon after the hinder part of the primitive streak becomes tucked in 

 on the ventral side of the embryo, an invagination may be noticed 

 where the tail of the embryo is folded off. This gradually becomes 

 deeper, and finally comes into contact with the hypoblast at the front 

 (primitively the apparent hind) border of the posterior section of the 

 primitive streak. An early stage in the invagination is shewn in the 

 diagram (fig. 106, an). It deserves to be noted that the anus lies 

 si uiie way in front of the blind end of the mesenteron, so that there 



