Chris Partridge wrote:
> Do not people talk about a pre-pubescent and pubescent stages of the body?
> Aren't there also fetal stages? What are these stages of? (01)
Yes, there are fetal stages. That is, the term 'stage' is extensively
used in embryology. There are developmental stages, and these are part
of the classification (a number of classifications, in fact) of
embryonic development. (02)
For example, Streeter (1942), and Müller and O'Rahilly (1987) classified
human embryonic development into 23 stages. Each stage is defined in
terms of either the structure of the embryo, or the processes it
participates in. (03)
For example: (04)
Stage 1. fertilized oocyte (1. day).
Stage 4. attaching blastocyst (days 5.-6.).
Stage 6. proliferation of extraembryonic mesoderm (days 13.-15.)
Stage 8. primitive pit, ntochondral canal (days 17.-19.)
Are these stages the states of the embryo, or are they the processes of
development of the embryo? Or are they perhaps rather the periods (time
intervals) in which the embryo has particular structures discernible and
over which the developmental processes happen? (06)
I am not willing to answer. But I think that you should not infer too
much from the mere expression 'the embryo is in stage 1.', as it may
mean either/both that it has a particular structure and that it is in
participating in a particular process. For example, stage 3. differs
from stage 2. in the structure. Stage 4. is distinguished from stage 3.
because during the former the blastocyst is attaching to the uterine
mucosa -- not due to structural differences in the blastocyst itself. (07)
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