322 



NATURE 



[June 15, 1916 



throat in some being whitish, in others black. The 

 bird now obtained from Tuskar Rock displays the 

 latter character in its plumage, and is indeed, the 

 black-throated wheatear {Saxicola stapazina) of 

 earlier writers. When I find time to compare 

 it I hope to be able to assign it to its 

 racial form. In the meantime it seems desir- 

 able to announce its occurrence without delay as a hird 

 quite new to Ireland. I have to express my great 

 gratitude to Mr. Glanville for so kindly sending me 

 this interesting specimen in the flesh for identification 

 and investigation. C. J. Patten. 



The University, Sheffield. 



EXPERIMENTAL BIOLOGY. 



WE use in our title the term Experimental 

 Biology, which requires some apology, as 

 a convenient label for an interesting bundle of 

 thirteen papers by Jacques Loeb and Hardolph 

 VVasteneys. They give an account of important 

 experiments bearing on a variety of puzzling bio- 

 logical problems, (i) Loeb showed many years 

 ago (1889) that some animals orient themselves 

 in relation to a luminous object so that their 

 plane of symmetry falls into the direction of the 

 rays of light, and suggested that this reaction 

 was comparable to the heliotropic reaction of 

 plants. In 1897 he brought forward evidence in 

 support of the view that the action of light in 

 evoking a heliotropic reaction is chemical, and 

 this theory is now confirmed by additional facts. 

 According to the law of Bunsen and Roscoe, 

 the photochemical effect of light is equal to the 

 product of the intensity into the duration of 

 illumination, and this has been shown to hold 

 for the heliotropic curvatures of plants (Blaauw 

 and Froschl) and of hydroids (Loeb and Ewald). 

 Furthermore, it has now been shown by Loeb 

 and Wasteneys that the region in the spectrum 

 most efficient in the production of heliotropic 

 curvature is almost the same for hydroids (Euden- 

 drium) and for oat seedlings. The investigators 

 suggest that there are two types of photosensitive 

 substance, one with a maximum sensitiveness (or 

 absorption) in the yellowish-green, and the other 

 with a maximum of sensitiveness in the blue. The 

 first type is represented by visual purple, and a 

 photosensitive substance of this type occurs in 

 Chlamydomonas (often claimed as a plant), in 

 Daphnia, and in many other organisms. The 

 second type of photosensitive substance occurs 

 in Euglena, in Eudendrium, and in many plants. 

 Thus the distribution of the type of substance 

 does not correspond to the boundaries between 

 plants and animals. 



(2) In another series of experiments Loeb in- 

 quires into the conditions which determine or 

 prevent the entrance of the spermatozoon into 

 the egg. It is well known that a fertilised egg 

 is non-receptive to other spermatozoa. What 

 is the nature of this block? It is not due to the 

 changes underlying the development of the egg, 

 for if the eggs of a sea-urchin are induced to 

 develop by the methods of artificial partheno- 

 genesis, a spermatozoon may still enter the egg 

 or an individual blastomere. By simply altering 

 NO. 2433, VOL. 97] 



the alkalinity of the sea-water Loeb can make a 

 sea-urchin ovum receptive or non-receptive to the 

 spermatozoon of a starfish ; this depends on some 

 rapid alteration of a physical property of the 

 surface of the ovum. And the ingeniously 

 worked-out experimental argument points to the 

 conclusion that a block of this sort is induced 

 when a spermatozoon fertilises an egg. 



But what of the more positive side of the 

 question? There is a widespread belief that a 

 spermatozoon shows a positive chemotropism for 

 the appropriate ovum, but Loeb finds no proof 

 of this in sea-urchins. The motility brings the 

 spermatozoon fortuitously near the egg; the 

 vibrations may assist in boring and in fixing the 

 spermatozoon to the surface of the ovum until 

 other forces, such as surface-tension, come into 

 play. What is certain is that the spermatozoon 

 cannot enter the egg unless physical conditions 

 at the boundaries of egg, spermatozoon, and sur- 

 rounding solution are right. It must be noted, 

 however, that a sea-urchin spermatozoon becomes 

 more active when it comes near an egg of its 

 own species, and Loeb suggests that this activat- 

 ing effect of the egg upon spermatozoa, being 

 most rapid as regards spermatozoa of its own 

 species, is a means of preventing hybridisation. 

 In other words, the activating influence of the 

 egg has some degree of selective specificity. 



(3) In a third set of experiments Loeb tackles- 

 the problem of the degenerate condition of the 

 eyes in some cave animals, such as fishes and 

 salamanders. Though a few zoologists cling to 

 the " natural " intei-pretation that the " blind- 

 ness," which differs considerably in degree, is 

 due to the hereditary accumulation of the results 

 of disuse, the difficulties in the way of accepting 

 this Lamarckian view are very serious. It has 

 been assumed, therefore, that the blindness of 

 some cave animals began as a germinal variation 

 or mutation. But confidence in the legitimacy 

 of this assumption has been lessened by the 

 meagreness of our knowledge as to the occur- 

 rence of variations in the direction of optic de- 

 generation. Very welcome, therefore, are Loeb's 

 recent experiments which show that degeneracy 

 of the eye can be readily induced by influences 

 affecting the condition of the egg or the earliest 

 stages of development. Thus, embryos with 

 degenerate eyes can be produced by fertilising 

 the eggs of Fundulus heteroclitus with the sper- 

 matozoa of Menidia. 



Since in these cases there is usually no circu- 

 lation in the feeble embryos, the inference is 

 suggested that the anomalous condition of the 

 eye may be due to lack of circulation. Blind 

 embryos of the pure breed of Fundulus may be 

 produced by the addition of KCN to the sea- 

 water ; and a short exposure of the fertilised ova 

 to temperatures between zero and 2° C. results 

 in abnormal embryos, a certain percentage of 

 which will show degenerate eyes. It is inter- 

 esting to learn that lack of light does not, in 

 the case of Fundulus, influence the development 

 of the eye. From Loeb's experiments it is not 

 to be argued that the blindness of cave animals 



