Experimental embryology / by Thomas Hunt Morgan.

  • Thomas Hunt Morgan
Date:
1927
Catalogue details

Licence: In copyright

Credit: Experimental embryology / by Thomas Hunt Morgan. Source: Wellcome Collection.

    103/790 page 85
    nucleus, a tetraster forms, and the first cleavage is into four equal cells (Fig. 95). The subsequent cleavages in these two kinds of eggs, as described by Dricsch (’92) and Morgan (’95) are as follows: At the second division of the three-fold type, the blastomeres divide by meridional planes into six cells (Fig. 10c). The next division is equatorial, forming two rings of six cells each (Fig. lOd, e). At the next division, each of the six antipolar cells gives rise to a micromere, while each of the six polar cells divides equally to Fig. 9.—Tetrapolar and tripolar spindles of dispermic and polyspermic eggs of the sea-urchin. (After O. and R. Hertwig.) produce twelve cells (Fig. 10/). It is obvious that the planes of cleavage follow the normal sequence, but in a three-fold instead of a two-fold pattern.1 In the other case—the four-fold type (Fig. 11&)—each of the first four blastomeres divides at the second cleavage in meri- dional planes (Fig. 115). There follows an equatorial cleavage i Trefoil eggs of Cerebratulus have been reported by Yatsu (’10). The second cleavage is through the pole, giving, as in the sea-urchin, a ring of six cells. The third cleavage is nearly equatorial (as in normal eggs). At the next cleavage the direction of the division is the same as at the normal, fourth cleavage. These trefoil eggs are probably due to dispermy. The fate of these eggs is not stated.
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