JrXE T, 1S95.] 



SCIENCE. 



627 



I 



merely a survival in philosophical hypothe- 

 sis of a pre-Aristotelian dens ex machina. 

 The dynamical hypothesis rejects the deu-i 

 ex machina, but finds a real mechanism in 

 the germ that is an automaton, but that is 

 such only in virtue of its structure and the 

 potential energy stored up within it. Every 

 step in the transformation of such a mech- 

 anism is mechanically conditioned within 

 limits by what has preceded it, and which 

 in turn so conditions within limits what is 

 to follow, and so on forever through a suc- 

 cession of descendants. The theory of 

 equipotential surfaces, as here applied to 

 organisms, leads to a theory of general mor- 

 phology that holds of all living forms, and 

 that is at the same time consistent with the 

 facts of development. 



EXPLAXATOKY XoTE TO PARAOKAPH OX PA(iE 61^. 



It now appears that the statement that the quarters 

 or eighths of an oiisperm are to be regarded as ' mole- 

 cular mechanisms of precisely the same type of po- 

 tentiality ' as the whole egg, must be taken with con- 

 siderable (lualiticatioii. Loeb (Ueber die Grenzeu 

 der Tlieilbarkeit dt-r Eisubstanz, Arcliir fiir Oes. 

 Phf/swhijie, vol. I.IX., 1-^94) has sho\^-n that the eggs 

 of echinoderms, if artificially divided, by means of a 

 method of his devising, into quarters or eighths, lose 

 the power of developing beyond the blastnla stage. 

 Tliis would appear to indicate that if the egg is sub- 

 divided so as to have its jwrts fall below a certain size, 

 these parts no longer have locked up within them, as 

 molecular mechanisms, as Loeb points out, enough 

 potential energy to transform themselves into com- 

 pletely equipped larva;. Or, perhaps, the initial 

 a^olotropy of the egg does not pennit of its subdivision 

 into (juarters and eighths without impairing their 

 structure and powers of development. 



My own recent experiments have shown that it is 

 possible to incubate for some time the genu of the 

 bird's egg ontside of the egg-shell in a covered glas.-;- 

 dish. These exi)eriments also show that restraints to 

 growth developed by the dying of a film of albumen 

 over the germ causes it to be most extraordinarily 

 folded, witli many abnormal tnmor-like growths from 

 both entoderm and ectoderm, that differ, however, in 

 histological character from the cells of both these 

 layers. Tliese experiments also prove that it is pos- 

 sible to mechanically divide the germ of tlie warm- 

 blooded Avian type into halves or <iuarters, and to 

 have these continue to develop for a time. 



Tlie convei-se of the process of mechanical division 

 of the germ \vc have in Borrt's remarkable experi- 

 ments in cutting recently-hatched Amphibian em- 

 bryos in two, and placing the separated halves again 

 in contact under such conditions as to cause them to 

 grow togetlier, or even to thus graft tlie half of a larva 

 of one species upon that of another. Tliat such gi-aft- 

 ingis po.ssible, I can testify, as a result of a repetition 

 of some of the experiments. See Bom's paper in 

 Schlfmscheit (jesdlsch. f. viilcrh'indische Ciilliir : Meili- 

 cini^che Section, l-i94. pp. 13. Supplementing Born's 

 results are Roux's experiments on cytotiopism, or the 

 reciprocal attraction of isolated blastomeres of Am- 

 phibian eggs {Archil- f. Entwickduiigsmecliatiil:, I., 

 1894), if brought close together, though at first not in 

 actual contact. Tliere is also some evidence of 

 asexual cdri/otropi.im as witnessed in the conjugating 

 nuclei of the cells of the intestinal epitlielium of land- 

 Isopods (Ryder and Pennington, Aniil. Anzeiyer, 

 1894). 



The experiments of 0. Schultzc (Anal. Ameiger, 

 Ergiinzungsheft zum Bd. IX., pp. 117-i:?2, 1894), by 

 very slowly rotating in a mechanically fixed position 

 the segmenting eggs of Amphibians on a specially 

 constructed clinostaf, with the result of disorganizing 

 and killing them, show that such eggs are not 

 isotropic. His production of double monsters in such 

 ova by disturbing, for a time, their geotropic relations, 

 is also significant, while his conversion of the mero- 

 blastic amphibian egg into a holoblastic, evenly seg- 

 menting one by merely rotating it through 180° out 

 of its normal geotropic relation, and allowing it -to 

 complete its segmentation in an inverted position, 

 proves that the egg can be made structurally homo- 

 genous by mere mechanical means, but at the expense 

 of its power to complete its development. This is 

 further proof that the egg is not isotropic in the sense 

 in which that word is used by natural philosophers. 



Since the appearance of the short but important 

 paper by Prof. E. B. Wilson and A. I'. Mathews 

 (Jour. ofMorphologn, Vol. X., Xo. 1, 1895), in which 

 they deny the existence of the centrosonie, it becomes 

 necessary for me to explain that the word 'centro- 

 sonie ' is used in the text in the sense in which they 

 use the expression 'attraction spheres.' Tlieir dis- 

 covery that the ovocenter, or attraction sphere of the 

 egg, disappears after the expulsion of the two polar 

 cells in echinotlenn eggs, to be replaced by the sperm- 

 center, is of the greatest significance, and may ex- 

 plain the rea.s<jn why jiarthenogenctic ^gs develop, 

 namely, as a conseipience of tlieir retention of an 

 ovocenter. Tlie new facts that these two able work- 

 ers have disclosed are entirely in harmony with a 

 dynamical theory of fertilization and sex. 



Jniix A. Ryder. 



