[34] According to Zur Strassen’s results the early embryology of Ascaris proceeds almost exclusively by cellular surface-changes: the most typical morphogenetic processes are carried out by the aid of this “means.” As a whole, the embryology of Ascaris stands quite apart and presents a great number of unsolved problems; unfortunately, the germ of this form has not been accessible to experiment hitherto.
[35] Rhumbler has recently published a general survey of all attempts to “explain” life, and morphogenesis in particular, in a physico-chemical way (“Aus dem Lückengebiet zwischen organismischer und anorganismischer Natur,” Ergeb. Anat. u. Entw.-gesch. 15, 1906). This very pessimistic survey is the more valuable as it is written by a convinced “mechanist.”
[36] Compare the analytical discussions of Klebs, to whom we owe a great series of important discoveries in the field of morphogenetic “means” in botany. (Willkürliche Entwickelungsänderungen bei Pflanzen, Jena, 1903; see also Biol. Centralblatt, vol. xxiv., 1904, and my reply to Klebs, ibid. 23, 1903.)
[37] Arch. Entw. Mech. 17, 1904.
[38] Zeitschr. wiss. Zool. 55, 1902; and Mitt. Neapel. 11, 1903.
[39] In certain cases part of the specific feature of the process in question may also depend on the “cause” which is localising it, e.g. in the galls of plants.
[40] Herbst, “Ueber die Bedeutung die Reizphysiologie für die kausale Auffassung von Vorgängen in der tierischen Ontogenese” (Biol. Centralblatt, vols. xiv., 1894, and xv., 1895); Formative Reize in der tierischen Ontogenese, Leipzig, 1901. These important papers must be studied by every one who wishes to become familiar with the subject. The present state of science is reviewed in my articles in the Ergebnisse der Anatomie und Entwickelungsgeschichte, vols. xi. and xiv., 1902 and 1905.
[41] Compare the important papers by J. Loeb, Untersuchungen zur physiologischen Morphologie der Tiere, Würzburg, 1891–2.
[42] I use the word “primordia” for the German “Anlage”; it is better than the word “rudiment,” as the latter may also serve to signify the very last stage of a certain formation that is disappearing (phylogenetically).
[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.