104 



EMBRYOLOGY 



at the posterior margin of the germ-disc of Reptiles and Birds, when 

 their gastrulation begins. An imagination proceeding from this 



point, such as DUVAL has 



A established for the Chick, 



is unfortunately not as 

 yet proven with sufficient 

 certainty in the case 

 of Mammals ; the origin 

 of the two -layered stage 

 is also still involved in 

 obscurity. 



However, there are in 

 the literature some observa- 

 tions, which, fragmentary 

 as they are, appear to me 

 to be worthy of special 

 regard. 



At the stage at which 

 the blastula has become 

 for a certain distance two- 

 layered (fig. 62), there has 

 been discovered by HEAPE 

 in the case of the Mole, by 

 SELENKA in the Opossum, 

 and by KEIBEL in the 

 "Rabbit, at one place of 

 the embryonic spot (pro- 

 bably in the region just 

 described as terminal ridge), 

 a small opening (fig. 64 u), 

 which is possibly to be in- 

 terpreted as Uastopore and 

 to be compared with tlie 

 crescentic groove of Birds. 

 Here the two primary germ- 

 layers are continuous with 

 each other, and from here, as well as from the primitive streak, the 

 middle germ-layer takes its origin. I assume that, beginning at 

 this place, the lower germ-layer has in a still earlier stage been 

 developed by an infolding of a small territory of the single-layered, 

 blastula (fig. 59). 



Fig. 62. -Blastula of the Eabbit 7 days old without the 

 outer egg- membranes. Length 4 '4 mm. After 

 KOLUKER. Magnified 10 diameters. 



Seen in A from above, in B from the side. 



ag, Embryonic spot (area embryonalis) ; ge, the line 

 up to which the blastula is two-layered. 



