486 THE HUMAN EMBRYO. 



The germinal layers of the human embryo. But little can 

 be said on this point at present. Concerning the mode of 

 establishment and of differentiation of the germinal layers, we 

 know nothing. In Reichert's ovum the outer wall of the vesicle 

 consists of a single layer of epithelial cells, which from Reichert's 

 figures appear to be flattened, or pavement cells, and which must 

 almost certainly be epiblastic. The central mass of cells, form- 

 ing the yolk-sac, is certainly hypoblastic ; and so also, in all 

 probability, is the thick disc of granular cells forming the deeper 

 layer of the embryonal area. 



Whether mesoblast is, or is not, present in Reichert's ovum 

 is a disputed point ; but at a stage not much later, in the 

 embryos E and SR, layers of vascular mesoblast are present, not 

 only in the embryo itself, but covering the outer surface of the 

 yolk-sac, and lining the wall of the blastodermic vesicle as well 

 (cf. Figs. 182, 183, and 184). The mode of origin, and the time 

 of appearance, of the mesoblast are unknown ; but from the 

 stage represented by the embryos E and SR, the history of the 

 layer is practically the same as in the rabbit or chick. 



3. The Third Week. 



During the third week, the embryo assumes more definite 

 form. The neural canal is closed along its whole length ; the 

 brain vesicles, optic vesicles, and auditory sacs are formed ; the 

 visceral arches and clefts develop ; and the head and neck 

 acquire their characteristic embryonic shape. The embryo in- 

 creases considerably in size ; the constriction between embryo 

 and yolk-sac becomes much more marked ; and towards the end 

 of the week the first rudiments of the limbs appear. 



During this time, development proceeds rather slowly, the 

 changes passed through in the course of the week corresponding 

 roughly to those effected during the second and third days in a 

 chick embryo. 



Only a limited number of embryos of the third week have 

 been described, not much more than a dozen in all, and only a 

 few of these were in satisfactory condition for detailed exami- 

 nation. Outlines of some of the more important specimens are 

 given in Figs. 189 to 195, and more detailed drawings on a 

 larger scale in Figs. 196, 197, and 198. 



