AVES. 



167 



manner as in Elasmobranchii, the Frog, etc., by the simple 

 conversion of an open groove into a closed canal. The closure 

 commences first of all in the region of the mid-brain, and 

 extends rapidly backwards and more slowly forwards. It is 

 completed in the Fowl by about the time that twelve meso- 

 blastic somites are formed. 



The mesoblast The general changes of this layer do not 

 exhibit any features of special interest the division into lateral 

 and vertebral plates, etc., being nearly the same as in the lower 

 forms. 



The hypoblast. The closure of the alimentary canal is 

 entirely effected by a process of tucking in or folding off of the 

 embryo from the yolk-sack. The general nature of the process 

 is seen in the diagrams figs. 105 and 121. The folds by which 

 it is effected are usually distinguished as the head-, the tail- and 

 the lateral folds. The head-fold (fig. 105) is the first to appear ; 



we. 



FIG. 105. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH 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-fold has not yet appeared. 



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



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

 alimentary tract; N.C. neural canal; C/i. notochord ; A. epiblast ; B. mesoblast; 

 C. hypoblast. 



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 formation 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 mesen- 

 teron. At the junction between the two folds there is present 



