400 WILSON. [Vol. VI. 



Nevertheless, although the form of cell-division is strictly 

 spiral up to the thirty-eight-celled stage, an adult bilaterality is 

 foreshadowed, long before the bilateral divisions begin, in the 

 arrangement of the cells. The anterior macromeres are sym- 

 metrically placed with reference to the future median plane, and 

 so in a certain sense are the primary micromeres, though this is 

 somewhat obscured by their rotation to the right. I may call 

 especial attention, however, to the fact that they gradually move 

 back into their original position {cf Figs. 20, 23, 28), and are 

 thus symmetrically placed, with reference to the median plane, 

 before the close of the spiral period. . This proves that the rota- 

 tion is bnt a temporary effect of mechanical conditions, and he7ice 

 has no morphological meaning. 



Up to the close of the spiral period, however, the embryologi- 

 cal material is not equally distributed on either side the future 

 median line, mainly on account of the storage of the substance 

 of the somatoblasts in the left posterior macromere. Upon the 

 separation and transference of this sttbstance to the middle line, 

 the asym^netry disappears, the posterior pair of entomeres being 

 equal, like the afitei'ior pair {cf Figs. 47, 48, 51, 55-57, ^6). 

 Immediately afterwards the bilateral divisions begin. This I 

 believe to be one of the most significant and important points in 

 the entire ontogeny. 



I. General History of the Upper Hemisphere. Origin of the 



Head-Kidney. 



The first bilateral division of the four primary micromeres 

 and the origin of the cross have been described at p. 395. Atten- 

 tion may be again called to the fact that the division of the pos- 

 terior micromeres {c^, d^) takes place some time before that of 

 the anterior pair {a^, b^). The second bilateral division of c^ and 

 d^ gives rise to a pair of cells, from which arise the head-kidneys ; 

 I shall therefore term them the cephalic nephroblasts. These 

 cells (Figs. 37, 38, c^-^, d^-^, colored orange) are smaller than c^ 

 and d^, and from the first differ from them in structure, the pro- 

 toplasm being clear and watery, and showing a marked tendency 

 to vacuolation. Shortly after this division of c^, d^, the anterior 

 pair of micromeres {a^, b^) divide in a similar manner (Figs. 39, 

 40), giving rise to a^-^ and b^-^ respectively. At this time the 



