316 PROCEEDINGS OP THE AMERICAN ACADEMY. 



evident from the fact that in the case of the bridge there is always 

 an opening leading from the outside into the cavity beneath the 

 bridge, and this cavity is never filled with amoeboid cells as is the 

 cavity in the mole. Furthermore, on the bridge there are found in 

 some cases " Deckzellen," — or rather their remains, consisting of 

 homogeneously staining nuclei and little cytoplasm, -^ which clearly 

 can take no part in the formation of either bridge cells or true 

 ectodermal cells. Again, the roof of overlying cells in the mole 

 makes its appearance and finally fuses with the germinal disk ectoderm 

 at an earlier stage in ontogeny than that at which the bridge in the 

 pig develops. 



The second theory, which seems to me the more probable, ho- 

 mologizes the bridge with the overgrowth along the dorsal side of 

 Amphioxus. The reasons for this comparison are that the two struc- 

 tures develop at about the same time in ontogeny in the two cases, 

 i. e. just after the formation of a didermic vesicle ; that, further, a 

 process of growth can be traced for the bridge which corresponds 

 closely to the method of growth in the case of the structure under 

 consideration in Amphioxus ; and, finally, that there is a median 

 thickening of the germinal disk corresponding topographically to the 

 medullary plate of Amphioxus, a free margin to the bridge corre- 

 sponding to the neuropore, and a canal at the opposite pole which 

 may, perhaps, be compared with the neurenteric canal of the primitive 

 vertebrate. From my present knowledge of the bridge in the pig, 

 I cannot homologize it with the roof-like structures in the shrew and 

 the mole, and if it is not comparable with the overgrowth in Amphi- 

 oxus, it seems to me necessary to regard it as a structure hitherto 

 undescribed in vertebrate embryology. Whatever the interpretation 

 of the bridge may be, we have the fact of its existence, and it is 

 reasonable to expect that it will be found in other mammals as well, 

 as, for example, in the sheep, in which the immediately succeeding 

 stages of development are so similar to those of the pig. 



I wish to express my thanks to Dr. Alexander Agassiz, to whom 

 I am greatly indebted for the opportunity of studying at his private 

 laboratory at Newport and for the privilege of using his library, and 

 to Dr. E. L. Mark, who has very kindly followed all my work and 

 examined my preparations with great care, and also to the employees 

 of the abattoir, who have given me much assistance in securing 

 material. 



Cam BRIDGE, April 18, 1894. 



