WEYSSE. — BLASTODERMIC VESICLE OF SUS SCROPA. 311 



2. Interpretation of the Bridge. 



I now have to consider the interpretation of the structure which I 

 have called the bridge. There are two structures which have been 

 described in vertebrate ontogeny with which it may perhaps be possi- 

 ble to compare it. One of these has been figured by Heape ('83, Plate 

 XXIX. Figs. 20-28) in the blastodermic vesicle of the mole. He here 

 shows a thickened ectoderm in the region of the germinal disk, with a 

 layer of entoderm beneath but not extending far beyond it, and above 

 it a cavity (his " secondary cavity ") which is roofed over by a bridge 

 of cells from the " Deckschicht," or, as he calls it, the " outer layer." 

 The history of the structure, as he gives it, is briefly this. The 

 blastodermic vesicle consists of a closed sac of flattened cells, the outer 

 layer, and of a mass of rounded cells within at one pole, the inner 

 mass. The latter differentiates into two parts, which become ectoderm 

 and entoderm. The ectoderm becomes continuous at its margin with 

 the outer layer, from which it is separated over its central area by a 

 shallow cavity. The ectoderm increases in extent and becomes some- 

 what cup-shaped, so that the cavity increases in depth, but it is filled 

 with amoeboid cells derived from the outer layer. Later the ectoderm 

 of the disk flattens out, and the cells of the outer layer above it fuse 

 with it and become a part of the true germinal ectoderm. The inter- 

 pretation which Heape puts on these phenomena is, that they are a 

 transitory representation of the inversion of the germinal layers which 

 is carried to such a great extent in some rodents. I see no reason 

 why Heape's observations and conclusions are not entirely correct. 

 A similar phenomenon has been figured by Hubrecht ('89, Plate XVI.) 

 in the hedgehog. Here, however, the portion of the outer layer, or 

 " trophoblast," which is separated from the germinal disk ectoderm, 

 does not become fused with the disk later, but remains in contact with 

 the uterine mucosa. This roof-like structure, where it comes in con- 

 tact with the ectoderm of the disk, is clearly continuous with the cells 

 of the " trophoblast " and also with those of the disk, so that Hubrecht's 

 ('89) Fig. 20 B, Plate XVI., resembles my figures of sections through 

 the posterior attachment of the bridge in the pig. 



The condition in the pig, although it seems at a casual glance to 

 resemble the structure in the mole embryo, is not directly comparable 

 with it. In the first place, the structure in the mole forms from the 

 beginning an uninterrupted covering to the ectoderm of the germinal 

 disk, and continues to do so through the subsequent development, up 

 to its complete obliteration through fusion with the disk. During the 



