DEVELOPMENT OP THE TWO PRIMARY GERM-LAYERS. 103 



of the layer. This on older eggs slowly spreads itself from the 

 embryonic spot toward the opposite pole, and thereby the whole 

 blastodermic vesicle gradually becomes two-layered. While this is 

 talcing place, changes also proceed at the embryonic spot, which has 

 become oval and somewhat larger. RAUBER'S layer disappears * 

 (fig. 61) ; the underlying cubical or spherical cells have become 

 cylindrical and more closely crowded together. Each of the primary 

 germ-layers is now composed of a single layer of cells. 



The two accompanying figures, which represent in two different 

 positions a Rabbit's egg seven days old, will serve for the illustration 

 of these conditions. In looking down from above (fig. 62 A) one sees 

 the embryonic spot (ag), now become oval. It is produced exclusively 

 by a definitely limited thickening of the outer germ- layer, and indi- 

 cateiTtEe place at which the cells are cylindrical ; in that respect it 

 corresponds to the embryonic shield of reptilian and avian embryos, 

 and is not to be confounded with the cell-plate (fig. 59), which was 

 described as a thickening of throne-layered blastula! In looking at 

 it from the side (fig. 62 5Jone~can distinguish on the blastula three 

 regions : (1) the embryonic spot (ag); (2) a region which includes the 

 upper half of the vesicle and reaches to the line ge, in which the wall 

 is still composed of two layers, but in which the cells of both the 

 outer and inner germ-layers are very much flattened ; and (3) a third 

 portion lying below the line ge, where the wall is composed exclusively 

 of the outer germ-layer. 



There now arises the important question, in what manner the two- 

 layered condition in Mammals arises out of the single-layered form. 

 One has reason to expect that gastrulation takes place here in 

 the same way as with the remaining Vertebrates, by means of an 

 invagination or an ingression of cells which proceeds from a definite 

 territory of the thickened cell-plate of the blastula ; in this con- 

 nection attention must be directed to the posterior end of the 

 embryonic spot. 



When the embryonic spot has acquired a pear-shaped appearance 

 (fig. 63), there is at its posterior end a somewhat less transparent, 

 because thickened, place (hw), which KOLLIKER has designated 

 the terminal ridge (Endwulst). It is comparable with the opacity 



* Two views are held concerning the manner in which RAUBER'S layer 

 disappears. According to BALFOUR and HEAPE, the flat cells become meta- 

 morphosed into cylindrical cells, which are interposed between the other 

 cylindrical cells ; according to KOLLIKER, on the contrary, they disintegrate 

 and disappear. 



