390 WILSON. [Vol. VI. 



since the protoplasm of the somatoblast assumes a brown- 

 ish tint. In either case the somatoblast can always be recognized 

 at a glance. In the macromeres the protoplasm is now differ- 

 entiated into two distinct portions, separated by a very definite 

 boundary, — an upper granular portion, in which the nucleus 

 lies, and a lower clear portion containing the fat-drops. The 

 boundary-line is very irregular, since the granular protoplasm 

 sends numerous pseudopodia-like processes into the clear sub- 

 stance. In living specimens deutoplasm-spheres can still be 

 seen in all of the cells, but they have diminished in number 

 and are less conspicuous than in earlier stages, 



5. The Fifth Cleavage (Figs. 19 to 26). 



Up to this point all of the blastomeres divide nearly simul- 

 taneously at each cleavage. Henceforward the divisions are no 

 longer synchronous throughout the embryo. The mosaic char- 

 acter of the development comes more and more plainly into 

 view, and it soon becomes necessary to consider the cleavage 

 not as a whole, but as a series of parallel cytogenies, each of 

 which takes its origin in a group of blastomeres as in a single 

 protoblast. A definite thirty-two-celled stage is, nevertheless, 

 attained, and we may therefore speak of a " fifth cleavage." 



The thirty-two-celled stage is attained by a series of steps 

 which follow a definite but not invariable order. Fii^st, the four 

 central micromeres {a^, b^, c^, d}) divide unequally in a left- 

 handed spiral (Fig. 19), thus giving rise to four smaller cells 

 (rt^-^, b^-'^, c^-"", d'^-'^) that alternate with the four trochoblasts 

 {a^-^, b^-^, c^-^, d^-^) and form with them an eight-celled girdle 

 that completely surrounds the central micromeres (Fig. 20). 

 Every stage of this division may be clearly observed in prepara- 

 tion by means of the nuclear figures, which are very large and 

 distinct. The spindles have from the first the spiral arrange- 

 ment shown in Fig. 19, and the position of the newly formed 

 cells is thus predetermined before the parent-cell shows any 

 external sign of constriction. 



Second, immediately following the last division (Fig. 20), or 

 sometimes simultaneously with it (Figs. 21, 22), the somato- 

 blast [X] buds off a small cell {x^) at its right-hand apex, and 

 at the same time the two posterior micromeres of the third 



