I08 WHITMAN. [VOL. I. 



I. CLEAVAGE, AND THE EARLY ESTABLISHMENT 

 OF BILATERALLY SYMMETRICAL RELATIONS. 



An accurate acquaintance with the principal events of cleav- 

 age is indispensable to a clear understanding of the derivation 

 of the germ-layers ; for these layers, and even the more impor- 

 tant organs arising from them, can be traced directly to special 

 blastomeres. 



First Period, ending with the Eight-cell Stage, — A sketch 

 of the cleavage, based upon results published in an earlier paper 

 (No, I, pp. 49-58, 75-76) is here introduced for the sake 

 of clearness in the discussion and descriptions which are to 

 follow. 



The first two cleavage-planes are vertical, dividing the egg into 

 four macromeres, three of which are relatively small, but nearly 

 equal. The fourth, larger segment, which contains the remnants 

 of the polar rings characteristic of this ^^^, occupies a posterior 

 position with reference to the future embryo, while the anterior 

 and the two lateral positions are held by the three smaller macro- 

 meres. The next step consists in the formation of four ectoblas- 

 tic micromeres, which eventually present the figure of a quarter- 

 foil at the animal pole. The posterior macromere (Diag, i, ;r) 

 first buds off the ectoblast xx, then the right macromere, h, 

 gives rise to the second ectoblast, bb, and immediately after- 

 ward the third and fourth ectoblasts, aa, and cc, are produced 

 simultaneously by the left macromere, a, and the anterior macro- 

 mere c. We arrive thus at the eight-cell stage so common 

 among worms and molluscs, represented by four macromeres 

 surmounted by four micromeres that lie in cruciform order 

 in the boundary-lines of the mother-cells. 



The complete orientation of this stage embraces some rela- 

 tions of position yet to be noticed. The alternation of the 

 micromeres with the macromeres is an arrangement brought 

 about by a rotation of the whole quarterfoil, each segment having 



(6,) Embryology of Clepsine. Quart Journ. Mic. Set., XVIII,, 1878, 



