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THE MECHANISM OF CELL‐DIVISION I. THE FORCES WHICH CONTROL THE FORM AND CLEAVAGE OF THE EGGS OF ECHINUS ESCULENTUS
Author(s) -
GRAY J.
Publication year - 1924
Publication title -
biological reviews
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 4.993
H-Index - 165
eISSN - 1469-185X
pISSN - 1464-7931
DOI - 10.1111/j.1469-185x.1924.tb00547.x
Subject(s) - cleavage (geology) , biology , microbiology and biotechnology , cleavage furrow , cell division , anatomy , biophysics , cytokinesis , cell , genetics , paleontology , fracture (geology)
Summary. 1. Soon after fertilisation the protoplasm at the surface of the egg of Echinus esculentus is differentiated off from the internal phase of the egg. The egg is thus divisible into two phases, the ectoplasm and the endoplasm. 2. Except during the process of cleavage fine processes of endoplasm extend out into the ectoplasm. 3. The outer surface of the ectoplasm is of a solid extensible nature. It is not markedly elastic. The underlying portion of the ectoplasm is fluid. 4. When cleavage occurs the ectoplasm tends to collect in the segmentation furrow and eventually divides the two portions of endoplasm from each other. 5. The normal form of the fully divided cell is due to the pressure exerted on the endoplasm by the ectoplasm. 6. The ectoplasm is only stable in the presence of calcium and potassium. It has different osmotic properties to those of the endoplasm. It rapidly shrinks in dilute acids. 7. During cleavage the normal form of the cleavage furrow is due to the force exerted by the ectoplasm against the force exerted by the asters. 8. The form and extent of the cleavage furrow depend on the position and size of the asters. The asters are capable of deformation by the ectoplasmic pressure. 9. Evidence is presented to show that the growth of the two asters must lead to a redistribution of the different phases of the protoplasm (peripheral endoplasm, and ectoplasm), such as is actually observed. Further, the final result of the enlargement of the asters must lead mechanically to normal cleavage. There are, thus, two essentials for cleavage, (i) the cell must be differentiated into two immiscible layers, ectoplasm and endoplasm, (ii) the cell must contain sufficiently large asters to bring about a redistribution of the endoplasm round these two centres. 10. The variations in the types of cleavage characteristic of other animal cells can be brought about in Echinoderm eggs by varying either the extent of the force exerted by the ectoplasm, or by varying the size or position of the two asters. 11. Cell‐division in Echinoderm eggs is the direct result of the asters reaching a critical size. In conclusion, I wish to express my gratitude to Mr R. Elmhurst, Director of the Marine Station, Millport, N.B., for providing me with facilities at Millport, and for his unfailing assistance in maintaining a constant supply of material during the period 1921‐24.