Me and My Zygote

Bill Vallicella of Maverick Philosopher has asked for assessments of an argument by Barry Smith and Berit Brogaard to the effect that zygotes are not substantially the same as the successor entitites that they (apparently) develop into. Bill summarizes the argument as follows:

1. The unicellular zygote is predestined to undergo fission.
2. Whatever undergoes fission ceases to exist at the moment of fission.
Therefore
3. The unicellular zygote will cease to exist at the moment of fission.
4. If a substance S ceases to exist at time t, then no substance S* existing at a time later than t is transtemporally identical to S.
5. The unicellular zygote is a substance.
6. A post-birth human being is a substance.
Therefore
7. No post-birth human being is transtemporally identical to a unicellular zygote.