ENTYPY OF THE GERMINAL AREA 31 



called Rauber's layer). No amnio-embryonic cavity appears between them. Soon 

 Rauber's layer disappears, and the embryonic ectoderm lies free on the upper 

 pole of the blastocyst. In some mammals, such as the mole, pig, and Tupaja, the 

 germinal area is for a short time distinctly inverted, as in Tarsius ; but the phases 

 resulting in the opening out of the blastoderm on to the surface are even more 

 distinctly seen than in that animal. In the mole, for instance (fig. 4-2, 6), the 

 cavity which hollows out the heart of the formative cell-mass is larger and deeper, 

 and is roofed in by the trophoblast for a time. The embryonic ectoderm is at 

 first markedly concave, but this is very soon rectified by the straightening of the 

 plate ; and the roof of the cavity disappearing, a phase is reached exactly like 

 that described for the rabbit after the disappearance of Eauber's layer. 



In another and considerable series of mammals, the inversion persists rather 

 longer, and the cavity never opens out on the surface of the blastocyst. but remains 

 roofed in by the trophoblast layer. This condition was named by Selenka 

 ' entypy of the germinal area.' 



FIG. 43. EMBRYONIC AREA OF MOLE IMMEDIATELY PRIOR TO APPEARANCE or PRIMITIVE STREAK 



AND FORMED OF TWO LAYERS ONLY. 



FIG. 44. EMBRYONIC AREA OF MOLE, SHOWING THE PRIMITIVE STREAK AND GROOVE ENDING 

 POSTERIORLY IN A CRESCENTIC THICKENING. 



The area is bilaminar in front, trilaminar in the posterior half. 



FlG. 45. A SOMEWHAT LATER STAGE IN WHICH THE PRIMITIVE STREAK REACHES TWO-THIRDS OF THE 

 LENGTH OF THE EMBRYONIC AREA, AND ENDS BEHIND IN A KNOB OR THICKENING. 



(Figs. 43, 44, and 45 are copied from Heape. They are magnified 49 diameters.) 



There are a number of variations in the manner in which the condition manifests itself. 



(A) In some of the bats (fig. 42, c) and in the hedgehog the cavity remains roofed in by 

 trophoblast, and persists as the amniotic cavity, the walls of the definitive amnion being formed 

 not by the folds as in the other group, but by upgrowth of cells on the inner surface of the 

 covering trophoblastic layer. 



(B) In mice and rats (fig. 42, d) the trophoblast over the formative cell-mass becomes greatly 

 thickened and invaginated into the interior of the blastocyte, necessarily pushing the mass before 

 it. A cavity appears in this mass of trophoblast (false amniotic cavity), which becomes continuous 

 with the primitive amnio-embryonic cavity. The whole blastocyst becoming tubular, the germinal 

 layers appear reversed, the entoderm being external to the ectoderm. In the further course of 

 development this persistent inversion of the germinal area is rectified by the tardy straightening 

 of the blastoderm and opening out of the amniotic cavity. 



(C) In the guinea-pig (fig. 42, e) the blastocyst is drawn out into a tubular shape just as in 

 rats and mice, and the formative cell-mass is inverted in the same fashion into its cavity. 

 The placental thickening of the trophoblast is not, however, invaginated to the same degree as 

 the formative cell-mass, so that direct connexion between them is lost. Accordingly, when the 

 amnio-embryonic cavity is formed, its roof is independent of the trophoblast ; it never opens 

 into the cavity in the interior of the trophoblast plug (false amniotic cavity), and it becomes 

 the definitive amnion. 



