And so we may close this section[43] on formative stimuli or “causes” of morphogenesis by shortly adding, more on account of its factual than of its logical interest, that the phenomenon of the determination of sex,[44] according to the latest researches, seems to depend on cytological events occurring in the very earliest embryonic stages, say even before ontogeny, and not on formative stimuli proper[45]: it seems, indeed, as if the sexual products themselves would account for the sex of the individual produced by them, particularly if there were differences in their chromatin.[46] [43] A full analysis of the subject would not only have to deal with formative stimuli as inaugurating morphogenetic processes, but also with those stimuli which terminate or stop the single acts of morphogenesis. But little is actually known about this topic, and therefore the reader must refer to my other publications. I will only say here, that the end of each single morphogenetic act may either be determined at the very beginning or occur as an actual stopping of a process which otherwise would go on for ever and ever; in the first case some terminating factors are included in the very nature of the morphogenetic act itself. [44] A full account of the present state of the subject will be found in Morgan’s *Experimental Zoology*, New York, 1907. [45] But there certainly exist many formative relations between the real sexual organs and the so-called secondary sexual characters. Herbst has given a full analytical discussion of all that is known on this subject; but the facts are much more complicated than is generally supposed, and do not lend themselves therefore to short description. See also Foges, *Pflüger’s Arch.* 93, 1902. [46] It seems that in some cases (*Dinophilus*, certain Arthropods) the sexual products are invariably determined as “arrenogennetic” or as “thelygennetic” (Wilson, *Journ. Exp. Zool.* ii. and iii. 1905-6), whilst in others (Amphibia) the state of maturation or “super”-maturation determines the sex of the future organism (R. Hertwig, *Verh. D. Zool. Ges.* 1905-7). δ. THE MORPHOGENETIC HARMONIES Let us now turn again to considerations of a more abstract kind: we have become acquainted with some morphogenetic interactions among the parts of a developing embryo; and, indeed, we can be sure that there exist far more of such interactions than we know at present. But it is far from being true that the development of each embryonic part depends on the existence or development of every other one.