While in fishes this conjugation of the male and female elements is usually ensured by the female casting her spawn into an artificial nest outside the body, on to which the male sheds his milt, in all animals (and, to some extent, birds, who occupy an intermediate position) there is an organic nest, or incubation chamber as Bland Sutton terms it, the womb, in the female body, wherein the fertilized egg may develop to a high degree of maturity sheltered from those manifold risks of the external world which make it necessary for the spawn of fishes to be so enormous in amount.  Since, however, men and women have descended from remote ancestors who, in the manner of aquatic creatures, exercised functions of sperm-extrusion and germ-extrusion that were exactly analogous in the two sexes, without any specialized female uterine organization, the early stages of human male and female foetal development still display the comparatively undifferentiated sexual organization of those remote ancestors, and during the first months of foetal life it is practically impossible to tell by the inspection of the genital regions whether the embryo would have developed into a man or into a woman.  If we examine the embryo at an early stage of development we see that the hind end is the body stalk, this stalk in later stages becoming part of the umbilical cord.  The urogenital region, formed by the rapid extension of the hind end beyond its original limit, which corresponds to what is later the umbilicus, develops mainly by the gradual differentiation of structures (the Wolffian and Muellerian bodies) which originally exist identically in both sexes.  This process of sexual differentiation is highly complex, so that it cannot yet be said that there is complete agreement among investigators as to its details.  When some irregularity or arrest of development occurs in the process we have one or other of the numerous malformations which may affect this region.  If the arrest occurs at a very early stage we may even find a condition of things which seems to approximate to that which normally exists in the adult reptilia.[75] Owing to the fact that both male and female organs develop from more primitive structures which were sexually undifferentiated, a fundamental analogy in the sexual organs of the sexes always remains; the developed organs of one sex exist as rudiments in the other sex; the testicles correspond to the ovaries; the female clitoris is the homologue of the male penis; the scrotum of one sex is the labia majora in the other sex, and so throughout, although it is not always possible at present to be quite certain in regard to these homologics.