20 AKT. 1. T. FUJITA : ON THE FORMATION 



THE SEVENTH QUARTET OF ECTOMERES AND THE CHANGE 

 FROM THE RADIAL TO THE BILATERAL, SYMMETRY. 



Siphonaria (a,.i.i, ^»,.1.1, Co.i.i, (/2.1.1, Fig. 20, PL I, and Fig. 21, 

 PI. II) : — Tlic seventh quartet of ectomeres is the lirst clescedaut of 

 the fifth (|uartet (^/,,.i, etc.). In Fig. 20, a horizontal section near 

 the vegetative pole, the first cell (L.,., of the new generation is 

 quite detached from the left posterior cell (/,-.i of the fifth 

 generation, which, however, is not seen in this figure being 

 situated more dorsally. Cleavage ]n-oceeds in a right-handed 

 spiral. The daughter-cell is quite similar in its general character 

 to the cells of the third, and the sixth generations. It lies ventral 

 to the mother-cell, and ultimately assumes a position alternate with 

 the cells of the fourth generation «;;, etc. (Fig. 24). From this 

 time on until the differentiation of the three germinal layers none 

 of the cells at the vegetative pole of the egg presents any marked 

 change. The vegetative pole is occupied hy the entomeres A, i/, 

 C, i>i, and the mesomere M surrounded l)y the ectomere-cells of 

 the fourth {a.;, etc.) and the seventh (c/j.i.ij titc.) generations (Figs. 

 20 and 24). 



Up to the [)resent stage each of the successive generations 

 of ectomeres has invariably been formed of a quartet, the cells 

 of which are produced in a certain regular way, and keep their 

 relative positions in a spiral form. Such a disposition of cells is 

 generally known as the spiral symmetry. This form of symmetry 

 is, however, generally not retained after the formation of the 

 mesomere-cell M, and is eventually changed into a bilateral sym- 

 metry. This, so far as 1 am aware, is said by investigators to be 

 caused, in most of the Molkiscan eggs simply by the new posi- 



