January 11, 1895.] 



SCIENCE. 



37 



like. Temperature has a profound efl'ect 

 not only on the rate of development, but 

 also on its form. Thus Driesch showed 

 that the eggs of sea-urchins when incu- 

 bated at a temperature sliglitly above the 

 normal undergo remarkable changes. The 

 form of cleavage may be considerably al- 

 tered (without affecting the end result of 

 development), and the gastrulation may be 

 profoundly affected. In some cases ' exo- 

 gastruhe ' are formed, the archenteron be- 

 ing turned out instead of in, and these 

 undergo all the normal differentiations of 

 the Pliiteu!', though they ultimately perish 

 since the alimentary canal is turned inside 

 out and the larva' are incapable of taking 

 food. Other physical agents siich as gravity 

 have been shown to have a profound effect 

 on development, determining the position 

 of roots and branches in hydroids (Loeb), 

 or even the polar differentiation of the egg 

 as in the frog (Ptliiger, Born, etc.). 



The most remarkalile and significant ex- 

 amples of environmental influence are, how- 

 ever, found in the effect of change in the 

 chemical environment. In the case of sea- 

 urchins Ponchet and Chabry found that in 

 sea-water deprived of calcareous matter the 

 Plutem larva is unal>le to develop its spicu- 

 lar skeleton, and Herbst showed that the 

 same result was produced hy a very slight 

 excess of potassium chloride in the water 

 even though the normal amount of calcare- 

 ous matter were present. In both cases the 

 larvic not onlj^ fail to develop spicules, but 

 are unable to produce the characteristic 

 ciliated arms. Thus arises a larva having 

 a simple ciliated belt and verj' .similar to 

 a j'oung Tornaria. This is a very instruct- 

 ive case : for it shows in the first place that 

 a definite character (formation of the skele- 

 ton) has a fundamental though verj- subtle 

 relation to the external environment, and 

 in the second place, that this relation indi- 

 i-ectly extends to other characters (ciliated 

 arms) that follow upon the development of 



the fii-st character. Such cases pave the 

 way to a rational conception of epigenesis. 

 by sliowing the multijilication of effecjts in 

 ontogeny and the complicated results that 

 may follow from a single and api)arently 

 insignificant condition of the environment. 



Even more striking results are those ob- 

 tained (by Herbst) by the addition of a 

 minute percentage of lithium chloride to 

 the sea-water. The primary result is to 

 cause exogastrulation (like the effect of 

 raised temperature). Beyond this, how- 

 ever, the entoderm area (i. e., archenteric re- 

 gion) often becomes abnormallj- large. The 

 entoderm may then be reduced to a mere 

 knob consisting of only a few cells, or may 

 even disappear altogether so that a blastula 

 is formed that conskts entirely of entoderm ! 

 This extraordinary result, if it can be ac- 

 cepted, shows that even so fundamental a 

 process as the differentiation of the germ- 

 layers stands in a vital relation to the 

 chemical environment. It is a revelation 

 of the importance of environmental influ- 

 ences in development and it shows that we 

 must readjust our conceptions not only of 

 development but also of inheritance, of 

 which development is an expression. Our 

 attention has been focussed too closely upon 

 the formal mori)hological aspect of develop- 

 ment which we have regiirded too largely 

 as the result of a prc-organized germ- 

 plasm operating like a machine. Embryo- 

 logical development must be thoroughly re- 

 examined from a physiological point of 

 view, full weight being given to the essen- 

 tial part played by the environment. This 

 point of view in no way sets aside the ne- 

 cessity of assuming a specifically oi-ganized 

 germ-i)lasm for every species as the basis 

 of inheritance. It simplifies the problem, 

 however, and opens the way for further in- 

 vestigation, which is practically barred liy 

 the artificial and foi-mal theories of devel- 

 opment advocated by Roux and Weismann. 



The third pajjcr read was by Professor 



