150 WALTER HE APE. 



simple or branched cell processes (fig. 47). At the points 

 where the cavity of the heart is continuous with the vessels 

 entering into and emanating from the heart, the epithelial 

 layer is continuous with the wall of these vessels. As I have 

 stated above, the heart shows no indication of curvature or of 

 division into chambers. 



The Blood-Corpuscles are formed from stellate mesoderm 

 cells. The nuclei of these cells become darker, the stellate 

 processes are then withdrawn and a meagre coating of proto- 

 plasm surrounds the now rounded nucleus. Such conditions 

 and changes are shown in many of the figures I have drawn ; 

 notably in fig. 25 in the heart, and in fig. 28 in the vitelline 

 vessels. 



Summary. 



External Features. — The early appearance of the optic 

 grooves (Stage e) which give rise to the optic vesicles ; the 

 existence of five visceral arches in Stage j ; the formation of 

 the amnion first at the hind end of the embryo ; and the 

 folding oflF of the head end of the embryo only, are the chief 

 points to be noted. The enclosure of the front end of the 

 primitive streak within the medullary fold ; the formation of 

 protovertebrae, chiefly from before backwards; the closure of 

 the medullary groove ; the appearance of three divisions of the 

 brain, and the formation of the heart are also detailed. 



The Epiblast. — The epiblast of the embryo (Stages e — g) 

 becomes formed into a median thickened portion, the medul- 

 lary plate, and into lateral portions which are formed of 

 cubical cells and are continuous with the flattened epiblast 

 cells which cover the vesicle. The closure of the medullary 

 groove (Stages h and j) causes the union of the lateral epiblast 

 which thus forms a continuous layer across the embryo. The 

 medullary groove commences about the centre of the embryo, 

 widening out into the sinus rhomboidalis behind and into the 

 cephalic plate anteriorly. The optic grooves are formed one 

 on each side of the middle line in the cephalic plate (figs. 4 

 and 16). 



