378 H. H. Newman and J. T. Patterson. 



The mesoderm is arising from the primitive streak region in 

 the characteristic manner, and laterally it thins out and, at the 

 point where the ectoderm turns up to give rise to the amnion, 

 divides into two layers, one following closely the amniotic ecto- 

 derm and the other the yolk-sac entoderm. 



Through the middle of the head process (fig. 18 h. p.) the ento- 

 derm at the center of the section is barely distinguishable from 

 the mesoderm, and in many places the union of these two layers 

 is very intimate. This must be looked upon however as a condi- 

 tion which is in all probability secondary. In the region of the 

 head process proper the mesoderm cells are closely packed to- 

 gether, but are entirely separate from the neural plate. 



Anterior to the head process the mesoderm rapidly thins out 

 practically to a single layer of cells and is easily distinguishable 

 from the entoderm (fig. 17). 



Anterior to this section the mesoderm passes into a thickened 

 region of the entoderm, which obviously has nothing to do with 

 the mesoderm, but owes its existence to a proliferation of ento- 

 derm cells (fig. 16, p. p. h.). It was not detected in the whole 

 mount preparations of the embryos, but its extent is easily deter- 

 mined by a study of sections. The thickening runs through the 

 first five sections beginning with the anterior tip of the embryonic 

 shield, and its width is equal to its length, and it therefore forms 

 a circular plate about 45 microns in diameter. In every respect 

 this circular spot corresponds to the "protochordal plate" of 

 Hubrecht, ('08), who has laid especial emphasis upon it as a re- 

 gion where the entoderm is clearly a source of mesoderm forma- 

 tion. Whatever may be one's conviction regarding Professor 

 Hubrecht's interpretation one can at least be certain that the 

 thickening is purely of entodermal origin in this species. Our 

 series is here too incomplete to permit of tracing out the 

 history of the protochordal plate, and thus to see whether its 

 definitive condition is simply that of mesoderm formation, or 

 whether it contributes to the formation of the fore-gut or the oral 

 plate. 



It should be stated here that the protochordal plate at the stage 

 under discussion thins out to a single layer towards its margin, 



