IV. i INTERACTIONS OF THE PARTS 271 



E. GODLEWSKI. Untersuchungen iiber die Bastardierung der Echi- 

 niden- und Grinoidenfamilie, Arch. Ent. Mech. xx, 1906. 



M. KKAHELSKA. Sur le developpement merogonique des ceufs de 

 Psammechinus, Bull. Internat. Acad. Sci. Cracovie, 1905. 



J. LOEB. Ueber die Befruchtung von Seeigeleiern durch Seestern- 

 samen, Pfluger's Arch, xcix, 1903. 



J. LOEB. Ueber die Reaction des Seewassers und die Rolle der Hydro- 

 xylionen bei der Befruchtung der Seeigeleier, Pfliiyer's Arch, xcix, 1903. 



J. LOEB. Weitere Versuche iiber heterogene Hybridisation bei Echino- 

 dermen, Pfliiger's Arch, civ, 1904. 



A. P. MATHEWS. The so-called cross- fertilization of Antenas by 

 Arbacia, Amer. Journ. Phys. vi, 1901-2. 



T. H. MORGAN. The fertilization of non-nucleated fragments of 

 Echinoderm eggs, Arch. Ent. Mech. ii, 1896. 



W. Roux. Ueber die Bedeutung der Kerntheilungsfiguren, Leipzig, 

 1883, Ges. Abh. 17. Leipzig, 1895. 



0. SEELIGER. Giebt es geschlechtlich erzeugte Organismen ohne 

 miitterliche Eigenschaften ? Arch. Ent. Mech. i, 1895. 



0. SEEHGEB. Bemerkungen iiber Bastardlarven der Seeigel, Arch. 

 Ent. Mech. iii, 1896. 



E. TEICHMANN. Ueber -Furchung befruchteter Seeigeleier ohne Be- 

 teiligung des Spermakerns, Jen. Zeitschr. xxxvii, 1903. 



H. M. VERNON. . The relations between the hybrid and parent forms 

 of Echinoid larvae, Phil. Trans. Roy. Soc. cxc, B. 1898. 



H. M. VERNON. Cross- fertilization among Echinoids, Arch. Ent. 

 Mech. ix, 1900. 



2. ON THE ACTION'S OF THE PAETS OP THE 

 DEVELOPING ORGANISM ON ONE ANOTHER 



Up to the present we have discussed only the original structure 

 of the fertilized ovum as an internal factor in differentiation. 

 We have now to consider to what extent the development of each 

 part may be due to influences exerted upon it by other portions 

 of the embryo. 



One of the first suggestions of such an interaction was put 

 forward by Oscar Hertwig. f We recognize ', he says in his 

 Zeit- und Streitfrage, ' apart from the nature of the egg-substance 

 itself, the conditions of development, first in those perpetually 

 changing mutual relations in which the cells of an organism are 

 placed to one another, and secondly in the influences exerted 

 upon the organism by the external world ' ; and again, ' physio- 

 logically expressed, the dissimilar differentiation of cells consists 

 in the reaction of organic substance to dissimilar stimuli/ and 



