152 EXTERNAL FACTORS III. 8 



fertilization, chlorine and sodium for segmentation, calcium for 

 the adequate cohesion of the blastomeres, potassium, calcium and 

 the hydroxyl ion for securing the internal osmotic pressure neces- 

 sary for growth, while without the sulph-ion and magnesium 

 the due differentiation of the alimentary tract and the proper 

 formation of the skeleton cannot occur; the secretion of pigment 

 depends on the presence of some sulphate and alkalinity, the 

 skeleton requires calcium carbonate, cilia will only beat in an 

 alkaline medium containing potassium and magnesium, and 

 muscles will only contract when potassium and calcium are there. 

 The part played by each substance is therefore specific ; for 

 some particular part of the morphogenetic process it is indis- 

 pensable. Not, of course, independently of internal factors, but 

 in co-operation with them, it does, in fact, determine the pro- 

 duction of organic form ; and the relation between the embryo 

 and the environment in which it develops is in this case, at any 

 rate, of the closest and most intimate kind. 



LITERATURE 



C. B. DAVENPORT and H. V. NEAL. Studies in morphogenesis. V. On 

 the acclimatization of organisms to poisonous chemical substances, Arch. 

 Ent. Mech. ii, 1896. 



C. FERE. A series of papers on the effect of various chemical bodies 

 on the development of the Chick, C. R. Soc. Biol. (9) v, 1893-liii, 1901. 



C. W. GREENE. On the relation of the inorganic salts of blood to the 

 automatic activity of a strip of ventricular muscle, Amer. Journ. PJiys. 

 ii, 1898-9. 



C. HERBST. Experimentelle Untersuchungen iiber den Einfluss der 

 veranderten chemischen Zusammensetzung des umgebenden Mediums 

 auf die Entwickelung der Tiere : I. Versuche an Seeigeleiern, Zeitsch. 

 wiss. ZooL Iv, 1893. 



C. HERBST. Experimentelle Untersuchungen iiber den Einfluss der 

 veranderten chemischen Zusammensetzung des umgebenden Mediums 

 auf die Entwickelung der Tiere : II. Weiteres iiber die niorphologische 

 Wirkung der Lithiumsalze und ihre theoretische Bedeutung, Mitt. Stat. 

 ZooL Neapel, xi, 1895. 



C. HERBST. Experimentelle Untersuchungen iiber den Einfluss der 

 veranderten chemischen Zusammensetzung des umgebenden Mediums 

 auf die Entwickelung der Tiere : III-VI, Arch. Ent. Mech. ii, 1896. 



C. HERBST. Ueber die zur Entwickelung der Seeigellarven nothwendigen 

 anorganischen Stoffe, ihre Rolle und ihre Vertretbarkeit : I. Die zur 

 Entwicklung nothwendigen anorganischen Stoffe, ArcJi. Ent. Mech. v, 1897. 



