One of the most conspicuous characteristics of the development of vertebrates is the adaptive specialization of pre-gastrular larvae. The eggs of vertebrates are laid upon land, in fresh water, in the sea, or within the mothers; the quantity of yolk they contain varies from the minute to the almost prodigious; they may develop at temperatures ranging from little above zero to little below 40° C., and at very different rates; the membraneous investments of egg and embryo are of widely different kinds; and so on. And accordingly we find that, although the strategy of pre-gastrular development is much the same in all vertebrates (inasmuch as all early development is directed towards the establishment of an axiate neurula of very uniform pattern), the overt tactics are in the very highest degree diverse. One need only reflect upon the variety of manœuvres that lead to the formation of endoderm, or upon the very different patterns of chorda-mesodermal invagination, to be convinced that this proposition is a truism.

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