DEVELOPMENT T mi; T\\U HM.MAKY I.I-:I:M LAI I"-'" 



of tin- layer. This on older e^s >lo\\ly Spread- ii> If from the 

 embryonic >pot to\\:trd tin- oppo>ite pole, and therel.v the whole 

 blaMi'drniiic \vsicle gradually becomes two-layered. While, this is 

 taking place, changes also proceed at the embryonic -pot. wliich baa 

 Income oval and somewhat larger. KAI r.i:u's layer di>appears* 

 dii:. 61); the underlying cubical or spherical cells have become 

 cylindrical and more closely crowded together. Kach of the prii 

 germ lay. is 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- 

 Cates the place at which th" cells are cylindrical ; in that respect it 

 corresponds to the embryonic shield of reptilian and aviari embryos, 

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

 described as a thickening of the one layered blastula. In l>kimj <it 

 it /rum the side (fig. 62 jB)one 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 

 i> >till 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 exelu>iv ly 

 of the outer germ-layer. 



There now ari-es the important question, in wl at manner the two- 

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

 One has reason to expect that gastrulat ion takes place h. re in 

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

 invalidation or an ingression of cells wliich proceeds from a definite 

 territory of the thickened cell-plate of the blast ula : in this con- 

 nection attention must he directed to the posteiior end of the 

 embryonic spot. 



When the embryonic spot has acijuin-d a pear-shaped appearance 



(fig. 63), there is at its post \ rior end a >omewhat l->s tiai:>paivnt. 

 1. cau.-e thickened, place (/<"'). which K<>U.IKI;U has -leMirnated 

 the terminal ridge (Kndwul.M). It is comparable with the opacity 



* Two views are held concerning the manner in wliich I 

 disappears. According to BALFOUR :inl UKAPI:. the Hat cells become meta- 

 morphosed into cylindrical cells, which are interposed between the other 

 cylindrical cells ; according to KI.LIKI:K, on the centruiy, they di>interrate 

 and dis-appear. 



