In his early development man is distinct from the other Primates, except that probably, judging from a single known stage in the chimpanzee, this distinctive feature of the early human embryo is also exhibited by the anthropoid apes. Whereas in the other Primates the embryo remains in the uterine cavity, no matter whether it corrodes the uterine wall for the formation of a placenta, as in Tarsius and monkeys, or does not form a placenta at all, as in the Lemuroidea, in man the early embryo penetrates completely into the endometrium and, covered by the decidua capsularis, it is altogether separated from the uterine cavity. This human characteristic occurs in some rodents also, but evidently there is no genetic relationship between man and the guinea-pig. Therefore, the guinea-pig is of no importance in endeavouring to understand the difference between man and the other Primates or even between man and the other mammals.
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