It is an elementary category error akin to stating the taste of the color blue to assert that a blastocyst is not a human being. The age of the human being is an incidental property of the human essence. You are confusing essence and accident.
...(of becoming a human being, in other words)
There is no such thing as a being becoming a human being. In the first place every living thing is thoroughly actual, with more or less potential, and in the second place potentialities or capabilities are limited to the kind of thing to which they belong.
As I noted above, your attempt to delineate a gamete as merely a "part" of a human and therefore free from protection fails the simplest of tests - can a human exist without this ostensible "part"? Indeed, if your reasoning is accepted, then the blastocyst itself is a mere "part" of a human being unentitled to protection, since an unimplanted blastocyst cannot alone become human
In addition to repeating the category error, you are confusing a principle of being with being itself, which is indicated by your use of quotation marks around "part". A principle of being is not itself a being, per se. It is a principle of being that gametes are a necessary condition for the existence of a human being, but that does not mean that a gamete is itself a human being or has or ever will have the capabilities of one. A gamete can never be anything other than itself. It has 23 chromosones, etc. It has the potential to fuse with another gamete to procreate a new human being, upon which it ceases to exist as such. A blastoscyst, on the other hand, is already a human being if it possesses the potency of ever doing things that are human, because potentialities and capabilities are limited to the kind of thing to which they belong. A sperm can only now or ever in the future do sperm things. A radish can only do radish things. No individual living body can "become" a human being unless it already is a a human being. No living being can become anything other than what it already essentially is.
Implantation is a necessary development for future growth of that human being-in-fact, but it has nothing to do with that human being's essence. The bottom line is that you are conflating the potency to cause something to come into existence with the potency for this new being to become fully what it is. That a child is exposed to the elements and starves to death so that he never reaches adulthood says nothing about his humanity. It only says something about certain accidental features of his existence, such as his age and state of development at his death. That a blastocyst never implants says nothing about his humanity. It says only certain things about the accidents such as his age and state of development at his death.
Cordially,
First, if a blastocyst is a "human being," then the human reproductive system is a deliberately designed holocaust. Second, this mystical nonsense about human "essence" is so utterly undefined (and undefinable) as to be perfectly meaningless.
There is no such thing as a being becoming a human being.
In which case, no line can be drawn, at blastocyts, gametes or prior.
Implantation is a necessary development for future growth of that human being-in-fact, but it has nothing to do with that human being's essence.
I'm afraid you're arguments, relying as they do on this mushy notion of "essence", are entirely arbitrary, and as such, entirely unsatisfactory.