File:Cytology, with special reference to the metazoan nucleus (1920) (20641792569).jpg

Title: Cytology, with special reference to the metazoan nucleus
Identifier: cytologywithspec00agar (find matches)
Year: 1920 (1920s)
Authors: Agar, Wilfred Eade, 1882-
Subjects: Cells
Publisher: London, Macmillan
Contributing Library: NCSU Libraries
Digitizing Sponsor: NCSU Libraries

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VI ECHINODERM HYBRIDS i6i circumstances it is not surprising that the embryos were of the purely maternal type. (b) We have already just emphasized that the nucleus can only exert its morphogenetic function through the cytoplasm. Now the earHer stages of development proceed with little or no increase of cyto- plasm. They consist indeed largely in the remodelling of the substance of the egg into that of the embryo. Hence in these very early stages the nucleus of the zygote has little opportunity to exert its morphogenetic function. This can only get full scope after the embryo has begun to form new cytoplasm by assimilation either of food substances supplied from without, or of reserve food material stored within it. Thus the
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Fig. 73. Diagram of the fertilization of the egg of Echinus microtuberculatus by the sperm of Myiilus galloprovin- cialis. (Kupelweiser, A. E-M., 1909.) A,'B, S nucleus, preceded by aster, approaching 9; C, D, first cleavajje nucleus formed entirely from the ? nucleus, i nucleus unchanged ; E, 2-cell stage. The S nucleus, still unchanged, lies inert in that one of the blastomeres to which cell division has chanced to relegate it. very early form of the embryo must be determined to a large extent by the physical constitution of the egg cytoplasm. The resemblance to the maternal species during cleavage, gastrulation, etc., which is brought about by the purely mechanical factors of size and number of yolk granules, viscosity of cytoplasm, etc., could only by a confusion of ideas be brought into the same category as the resemblance to parents due to the presence of tln^ living, self-repro- ducing idioplasm. To apply the word " Heredity " to both these cases would be to confuse the meanings of the term as used biologically and socially, as is done when a child who has been infected in utcro with Spirochaeta pallida is said to have inherited the disease of syphilis. Finally, it must be mentioned that the only Echinoderm hybrids which have been examined in the adult condition, namely, the offspring of the crosses Echinus miliaris ^ x Echinus acuius (^ (Shearer, de Morgan

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