96 PROF. ALLMAN ON THE 



a vertical axis, while from that end of each which is turned towards 

 the " actinal " pole of the ovum, or the pole where the mouth is 

 ultimately to show itself, a small segment becomes constricted off. 

 In the eight small spheres thus resulting, segmentation now sets 

 in with great activity ; these spheres multiply rapidly by division ; 

 and it is the layer of cells so formed which gradually extends 

 towards the opposite pole of the egg over what remains of the 

 eight large spheres , until these are finally completely enveloped 

 by it. The large spheres also become multiplied by division, but 

 much more slowly than the small ones. 



The enveloping layer thus formed he calls the embryonal layer, 

 and regards it as that from which the embryo is directly deve- 

 loped, while he regards the included spheres as performing only 

 the functions of a food-yolk. This view, however, I find it as 

 difficult to bring into harmony with my own observations cited 

 above as I do the somewhat different views of Kowalewsky re- 

 garding a formative and a food-yolk *. It appeared to me certain 

 that the mass of central spheres is the source from which the 

 whole vascular system of Beroe is directly developed f. Indeed 

 this, as has been since especially insisted on by Chun, is to become 

 the endoderm, while the peripheral layer is the foundation of the 

 ectoderm. 



Agassiz has also seen the commencement of the swimming- 

 plates in local accumulations of the cells which constitute the 

 outer layer, and has recognized the formation of the mouth in a 

 depression which takes place in a thickening of this layer at one 

 pole, while the sense-body originates in a similar thickening at 

 the opposite pole. He has further followed the formation of the 

 digestive cavity, which shows itself as a deepening of the oral 

 depression, accompanied by an inversion of the outer layer, the in- 

 bulging thus formed being describedas gradually extending through 

 the axis until it reaches the walls of the embryo at the opposite 

 pole. He does not appear, however, sufficiently to insist on the 

 fact that the tube formed by the inversion of the superficial layer 

 always opens into a cavity which had been already formed in the 

 centre of the embryo. I regard the independent formation of 

 this central lacuna, and the opening into it of the digestive canal, 

 as one of the most important morphological facts in the embryo- 

 logy of the Ctenophora. 



* See above, p. 92. t See above, p. 91. 



