314 



EMBRYOLOGY. 



heart, and formed the involucrum capitis ; so, towards the pos- 

 terior part, it has hent over as the involucrum caudse, and been 

 formed into plaits or folds laterally, as the lateral envelopes. 

 Thus is the serous layer of the germinal membrane, or upper 

 layer of the ventral laminae, raised on every side to converge 

 into an elliptical plait towards the back of the embryo ; on the 

 fourth day, these plaits have approached each other very closely ; 



a 



i--/ 



y } fa, 



~^r- /-. 





Fig. 343. Ideal section of an embryo nearly at the end of the third 

 day : A, transverse section ; a, vitelline membrane, &, b, laminae dorsales, 

 &c., as in fig. 338. B, longitudinal section. The cranial and caudal in- 

 volucra approximate, and at length meeting, they close the arnnion; g, the 

 eye ; k, entrance into the mouth, or fovea cardiaca ; i, the oesophagus, 

 with the rudimentary lung budding out as a diverticulum from it ; k, ex- 

 pansion of the alimentary tract, marking the seat of the stomach ; /, pos- 

 terior shut extremity of the intestine, from which proceeds the allantois, e, 

 surrounded by the vascular lamina dj m, the mesenteric lamina ; n, pas- 

 sage from the vitellus to the open abdomen ; 0, anterior part of the head 

 (corpora quadrigemina) ; p, hemispheres ; r, superior maxilla ; s, inferior 

 maxilla ; I, oral cleft or aperture ; 1, 2, 3, three branchial clefts. Other 

 references as in fig. 338. 



