DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 135 



like the one of the Rabbit here figured to be mistaken for it. In 

 the case of the human embryo, one sees a deep-cutting primitive 

 groove, and at the easily recognisable blastoporic lip (id) the bend- 

 ing over of the outer germ-layer (ak) into the parietal lamella (mk 1 ). 

 The visceral lamella (mk 2 ) is well separated from the latter for some 

 distance; under the primitive groove it is merged with the inner 

 germ-layer, the edges of the potential folds of the two sides being 

 fused into a mass of cells, which forms the floor of the primitive 

 groove,. 



Finally an agreement with the development of the Amphibia is 

 not wanting in sections which are made through the embryonic 

 areas of Birds, Reptiles, and Mammals behind the primitive groove. 

 The middle germ-layer begins to spread itself out backward also, 

 not, however, as in the anterior part of the embryonic area, in 

 the form of paired fundaments, but rather as a single continuous 

 cell-mass. This outgrowth too is united to the two primary 

 germ-layers only in the region of the posterior end of the primi- 

 tive streak, being elsewhere distinctly separated from both of 

 them. 



For the completion of the previous account, some statements 

 about the further growth of the middle germ-layer may now be 

 added, concerning which cross sections through embryos of various 

 ages afford evidence. The middle germ-layer spreads itself out 

 on all sides between the two primary germ-layers, farther and 

 farther from the place of its first formation the vicinity of the 

 primitive groove. At first it is limited to the fundament of the 

 embryo itself, then it makes its way into the area pellucida, and, 

 finally, it is encountered in the opaque area. Everywhere and 

 constantly in its extension it appears as an entirely independent 

 layer, at least two cells thick, which is separated from its surround- 

 ings by fissures. It is found to be united for a short distance with 

 the inner and outer germ -layers, but only at the primitive groove, 

 which persists for a long time, in older embryos even, as we havo 

 already learned from surface-views. Even in the stage when the 

 neurenteric canal traverses the primitive streak, and puts the 

 ccelenteric cavity (under the entoderm, fig. 100 liy] in communication 

 with the neural tube, we see the cellular lining of the canal and the 

 middle germ-layer fused, so that in this region a connection still 

 exists between all three germinal layers. Compare the accompany- 

 ing cross sections through embryos of Lacerta muralis. 



After the statement of the actual conditions, the questions remain 



