QUANTITATIVE AND QUALITATIVE CLEAVAGES 



335 



Isolation of blastomeres in the eight-cell stage of development, in most cases, 

 results in abnormal development. 



In Amphioxus, as mentioned previously, isolation of the first two blasto- 

 meres results in the production of twin embryos of half the normal size. In 

 the eight-cell stage in Amphioxus, the isolated smaller micromeres will de- 

 velop blastulae of ectoderm only, whereas the macromeres will develop blas- 

 tulae with developed entoderm, mesoderm, and ectoderm. In the four-cell 

 stage, if the two posterior blastomeres are separated from the two anterior 

 blastomeres, the former develop early embryos which have entoderm and 

 mesoderm together with ectoderm; the latter have notochord and neural plate 

 together with ectoderm and possibly a little of the mesoderm (Conklin, '33). 

 Similarly, in the frog or in the newt, when the first cleavage plane bisects 

 the gray crescent, the isolation of the first luo blastomeres results in the 



Fig. 166. Nuclear equality in the sea-urchin egg. (.AD) Normal cleavage. While 

 nuclei and black nuclei theoretically so designed to show nuclei in animal and vegetal pole 

 cells respectively. (E) Four-cell stage flattened by pressure, showing position of spindles 

 for the third cleavage parallel \.o pressure surface. (F) Fight-cell stage under pressure. 

 Compare with (C). normal. (G) Horizontal cleavage resulting from release of pressure 

 after eight-cell stage. Note mixed distribution of nuclei. Later development normal, with 

 cytoplasmic, organ-forming substances determining development as in fig. 163. Thus it 

 appears that the nuclei are equal within the blastomeres. whereas the cytoplasm is un- 

 equally (i.e.. qualitatively) distributed to the respective blastomeres, the particular type of 

 development of the blastomeres being dependent upon the cytoplasmic substance present. 



Black cytoplasm = micromeres which form primary mesenchyme. Coarse dotting = 

 entoderm, secondary mesenchyme and coelomic material. White, light stipple, and vertical 

 lines = ectodermal cells. 



