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Fig. 1.31 The bilaminar membrane of the cell forms the original pattern for the double-bag image, which is repeated over and over
again in macro-anatomy. (Reproduced with kind permission from Williams 1995.)
Fig. 1.32 The mucousy zona pellucida surrounds the ovum, and continues as an organismic membrane around the morula and
blastocyst until it thins and disintegrates at the end of the first week of embryonic development as the blastocyst expands, differentiates,
and prepares for implantation.
between 50 and 1000 of the fastest sperm beat their
heads uselessly against the zona pellucida, making
pockmarks with the hyaluronidase in their heads (and
dying) until some lucky slowpoke comes along and
comes in contact with the cell membrane itself and does
the actual fertilization.
When the fertilized egg divides, it is this zona pellu-
cida that contains the zygote (Fig 1.33A). The huge size
of the original ovum allows it to divide again and again
within the zona pellucida, and each successive set of
cells takes up nearly the same amount of space as the
original large cell. Thus this 'ground substance' shell
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