730 



NA TUHE 



[November 17, 1923 



Some Developments of Modern Zoology.* 

 By Prof. J. H. Ashworth, D.Sc., F.R.S. 



ZOOLOGY has fur out^'rown its early Iwundaries 

 wlien it could be defined simply as a part of 

 natural history, and at no period has its growth been 

 more rapid or more productive in results of scientific 

 and practical imjx>rtance than during the last two or 

 three decades. That period has witnessed a growth 

 of our knowledge of the living organism of the same 

 order of importance as the progress in our knowledge 

 of the atom. Never have investigators probed so 

 deeply or with so much insight into the fundamental 

 pr()l)lems of the living animal ; the means for observa- 

 tion and recording have become more delicate, and 

 technique of all kinds more perfect, so that we can 

 perceive details of structure and follow manifestations 

 of activity of the organism which escaped our pre- 

 decessors. 



Among the notable features of zoological activity 

 during the last twenty-five years, the amount of work 

 on the physiology of organisms other than mammals 

 must attract early notice in any general survey of 

 the period. Eighty years ago Johannes Miiller's 

 physiological work was largely from the comparative 

 point of view, but for some years after his death the 

 comparative method fell into disuse, and the science of 

 I)hysiology was concerned chiefly with the mode of 

 action of the organs of man or of animals closely related 

 to man, the results of which have been of outstanding 

 importance from their bearing on medicine. Interest 

 in the more general applications of physiology was 

 revived by Claude Bernard (" Lemons sur les pheno- 

 m^nes de la vie," 1878), and the appearance of Max 

 Verwom's " General Physiology," in 1894, was in no 

 inconsiderable measure responsible for the rapid exten- 

 sion of physiological methods of inquir\' to the lower 

 organisms — a development which has led to advances 

 of fundamental importance. Many marine and fresh- 

 water organisms lend themselves more readily than the 

 higher vertebrates to experimentation on the effects 

 of alterations in the surrounding medium, on changes 

 in metabolic activity, on the problems of fertilisation 

 and early development, on the chemistry' of growth 

 and decline, and to the direct observation of the 

 functioning of the individual organs and of the effects 

 thereon of different kinds of stimuli. The study of 

 these phenomena has greatly modified our interpreta- 

 tion of the responses of animals and has given a new 

 impetus to the investigation of the biology and habits 

 of animals, i.e. animal behaviour. 



This line of work — represented in the past by notable 

 contributions such as those by Darwin on earth- 

 worms, and by Lubbock on ants, bees, and wasps — 

 has assumed during the last two or three decades a 

 more intensive form, and has afforded a more adequate 

 idea of the living organism as a working entity, and 

 revealed the delicacy of balance which exists between 

 structure, activity, and environment. 



The penetrating light of modern investigation is 

 being directed into the organism from its earliest stage. 

 During the summer of 1897 Morgan discovered that 



' From the presidential address delivered to Section D (Zoology) of the 

 British Association at Liveipool on September 13. 



NO. 2820, VOL. I 12] 



the eggs of sea-urchins when placed in a 2 per cent, 

 .solution of sodium chloride m sea-water and thf n 

 transferred to ordinarv* sea-water would 

 cleavage and give rise to larvae, and J. Loeb's II 

 tions in this field are familiar to all students of zool- 

 Artificial parthenogenesis is not restricted to the « . 

 of invertebrates, for Loeb and others have shown tit 

 the eggs of frogs may be made to develop by prif kn.- 

 them with a needle, and from such eggs frogs have bt i n 

 reared until they were fourteen months old. The 

 application of the methods of micro-di.ssertion to the 

 eggs of sea-urchins is leading to a fuller ■ •• of 



the constitution of the egg, of the method « vtion 



of the sperm, and of the nuclear and lytupia^mic 

 phenomena accompanying maturation and fertilisation, 

 and will no doubt be pursued with the object of arriving 

 at a still closer analysis of the details of fertihsation. 



The desire for more minute examination of develop- 

 ing embryos led to the more careful study of the egg- 

 cleavage, so that in cases suitable for this method of 

 investigation each blastomere and its products were 

 followed throughout development, and thus the in- 

 dividual share of the blastomere in the cellular genesis 

 of the \arious parts of the body was traced. This 

 method had been introduced by Whitman in his thesis 

 on Clepsine (1878), but it was not until after the classical 

 papers of Boveri on Ascaris (1892) and E. B. Wilson 

 on Nereis (1892) that it came into extensive use. For 

 the next twelve or fifteen years, elaborate studies on 

 cell-lineage formed a feature of zoological literature 

 and afforded precise evidence on the mode of origin 

 of the organs and tissues, especially of worms, molluscs, 

 and ascidians. A further result of the intensive study 

 of egg-cleavage has been to bring into prominence the 

 distinction between soma-cells and germ-cells, which 

 in some animals is recognisable at a ven*' early stage, 

 e.g. in Miastor at the eight-cell stage. The evidence 

 from this and other animals exhibiting early segrega- 

 tion of germ-cells supports the view that there is a 

 germ-path and a continuity of germ-cells, but the 

 advocates of this view are constrained to admit there 

 are many cases in which up to the present an indication 

 of the early differentiation of the germ-cells has not 

 been forthcoming on investigation, and that the prin- 

 ciple cannot be held to be generally established. 



A cognate line of progress which has issued from the 

 intensive study of the egg and its development is 

 experimental embr}olog\- — devoted to the experimental 

 investigation of the physical and chemical conditions 

 which underlie the transformation of the egg into 

 embryo and adult. By altering first one anti then 

 another condition our knowledge of development has 

 been greatly extended. By artificial separation of the 

 blastomeres the power of adjustment and regulation 

 during development has been investigated, and by 

 further exploration of the nature of the t^% the presence 

 of substances foreshadowing the relative proportions 

 and positions of future organs has been revealed in 

 certain cases, the most striking of which is the egg of 

 the Ascidian Cynthia partita (Conklin, 1905). 

 Progress in in\"estigation of the egg has been 



