116 WALTER HEAPS. 



of the crescent being projected upwards and somewhat closely 

 approximated to the lateral epiblast of the embryo (figs. 19, 

 23, and 24). 



The epithelium of the dorsal border of the sac is thinner 

 than that of the ventral border, the difference being more 

 apparent in the hinder portion than in the front portion of 

 the sac. The points of the lateral horns are lined with 

 cylindrical cells. 



There is no distinct evidence at this stage (h) of outgrowths 

 of the fore-gut iu the position of the future visceral arches, 

 but slight indications of the invagination of the epiblast may 

 be seen corresponding to the grooves mentioned in the descrip- 

 tion of the surface view of an embryo of Stage h. 



On the ventral surface at the anterior end of the fore-gut in 

 Stages G and h (figs. 19 and 22J two slight invaginations of 

 the epiblast may be seen one on either side of the middle line, 

 and a few sections further backwards the epiblast and hypo- 

 blast are closely applied in the middle line, and there is a deep 

 median groove in the epiblast (fig. 23). 



At the close of Stage j there is a still further change in these 

 relations. The lateral outgrowths of the fore-gut are now 

 directed towards invaginations of the epiblast which corre- 

 spond to the grooves mentioned in the description of a surface 

 view of an embryo of this stage (Stage j). The outgrowths 

 are directed outwards and downwards from the lateral portions 

 of the lumen of the canal (fig. 46). The hypoblast and epiblast 

 have met and are partially fused in the case of the anterior 

 diverticula, although there is as yet no perforation consti- 

 tuting a definite cleft, but iu the more posterior diverticula 

 the hypoblast does not meet the epiblastic involution. 



Now also the fore-gut is a little louger, and the fusion of 

 epiblast and hypoblast on the ventral surface near the front end 

 is closer, although the perforation to form the mouth has not 

 yet taken place (fig. 49). 



This invagination of the epiblast is clearly seen in an embryo 

 of this stage to be in the form, anteriorly, of two shallow 

 grooves which converge posteriorly, these forming a deep 



