100 PEOF. ALLMAN OS THE 



layers of cells and its central cavity, and with one pole closed 

 while the opposite pole remains open, Chun has recognized the 

 morphological significance of a gastrula, and sees in the two 

 layers of cells which form its walls the two primitiye germinal 

 layers, ectoderm and endoderm. He thus takes a view very 

 diiferent from that of Kowalewsky and A. Agassiz, who regard 

 the large central cells as having the physiological value of a food- 

 yolk. My own observations on the development of JBeroe, as 

 cited above, lead me to conclusions quite in accordance with the 

 view taken by Chun. 



The central cavity still communicates with the exterior by the 

 gastrula-mouth, but becomes narrower by the pressure exerted 

 by the ectoderm-cells, and is at last reduced to the condition of 

 a narrow slit. In the meantime the ectoderm layer has been 

 extending itself concentrically over the open pole, which it 

 finally completely covers, and thus permanently closes the gas- 

 trula-mouth. 



After the ectoderm has in this way completely grown over the 

 endodermal cells, and both the poles have become closed, a 

 thickening of the ectoderm may be noticed on each pole, caused 

 on that where the gastrula-mouth had been situated by the cells 

 here acquiring an elongated cylinder shape, while on the opposite 

 pole the thickening is the result of the multiplication of the 

 cells, which here become disposed in several layers. Out of the 

 elongated ectoderm-cells which have shown themselves on the 

 pole previously occupied by the gastrula-mouth, the central 

 nervous system is to be diff'erentiated, while in the thickened 

 ectoderm of the opposite pole an inbulging may now be noticed ; 

 and we thus obtain the definitive mouth and the foundation of 

 the digestive canal. 



With this stage we have the first data for the orientation of 

 the embryo ; for we can now distinguish an oral pole from the 

 opposite sense-pole, the axis passing through the two poles being 

 the main axis of the Ctenophore. 



Simultaneously with the accumulation of ectoderm-cells on the 

 oral pole a similar accumulation may be seen in all Ctenophores, 

 except the Beroidse, on the right and left half of the embryo. 

 The paired protuberances thus formed represent the foundation 

 of the tentacle-apparatus. 



Yery early four symmetrically placed groups of ectoderm-cells 

 become difierentiated in a meridional direction from the rest by 



