5 90 WILSON. [ Vo L. V 1 1 1 . 



9. The i6-celled stages when shaken give rise to an 

 immense variety of forms which I have not carefully studied. 

 The isolated blastomeres continue to divide for some time, but 

 as far as observed always give rise merely to flattened plates or 

 shapeless masses of cells. Small closed ciliated blastulas were 

 also produced of about the same size as those derived from the 

 Yi^ blastomeres. 



B. Cleavage of the Isolated Blastomeres. 



The isolated blastomere retains for a short time its original, 

 flattened form (Figs, yj, 87). It soon becomes more rounded 

 (Figs. J'S, ^?i) but not perfectly spherical, and the first division 

 (invariably equal) was in every case observed transverse to the 

 longest diameter. It is an interesting fact that soon after their 

 isolation the blastomeres frequently explode with such violence 

 as to reduce them to minute granules, so that I have often 

 found scarcely a trace of them in the hanging drop where they 

 had carefully been isolated. The explosion, which I have 

 witnessed several times, is sudden and complete, and shows 

 that the substance of the blastomere is in a state of high 

 tension. 



The isolated blastomeres undergo a cleavage that approxi- 

 mates more or less nearly to that of a normal ovum, but tJie 

 extent of divergence is nearly proportional to the age of the 

 initial form. 



a. The Yi blastomere in many cases agrees exactly with the 

 normal ovum up to the 8-celled stage (Figs. 77-81). In later 

 stages, likewise, the general history is the same (Figs. 97-100) 

 though the bilaterality is usually lost, and a cleavage-pore is 

 more usually present. In many cases, however, the second 

 cleavage is distinctly unequal, so that in the 4- and 8-celled 

 stages (Figs. 90, 104, 105) two of the macromeres are smaller 

 than the others. The four micromeres appear to be always 

 equal. In later stages these embryos agree with the others, 

 except in the somewhat greater size of the macromeres towards 

 one side of the entoblast plate. 



b. The Y\ blastomere may likewise segment quite like the 

 entire ovum (Figs. 87-91), but the unequal type of the 4-celled 



