D.—ZOOLOGY. 111 
The desire for more minute examination of developing 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 individual share of the 
blastomere in the cellular genesis of the various 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. About the time of our last meeting here, and 
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 
very early stage, e.g. in Miastor at the eight-cell stage. The evidence 
from this and other animals exhibiting early segregation of germ-cells 
supports the view that there is a germ-path and a continuity of germ- 
ceils, 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 differen- 
tiation of the germ-cells has not been forthcoming on investigation, and 
that the principle cannot be held to be generally established. 
A cognate line of progress which, during the period under review, 
has issued from the intensive study of the egg and its development is 
experimental embryology—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 and 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 egg the presence of substances fore- 
shadowing the relative proportions and positions of future organs has 
been revealed in certain cases, the most striking of which is the ege of 
the Ascidian Cynthia partita (Conklin, 1905). Still further intensive 
study of the cytoplasm and nuclei of eggs and cleavage stages is required 
to throw light on the many problems which remain unsolved in this 
domain. 
Progress in investigation of the egg has been paralleled by increase 
in our knowledge of the germ-cells, especially during their maturation 
into eggs and sperms, the utmost refinements of technique and observa- 
tion having been brought to bear on these and on other cells. During 
the last thirty years, and especially during the latter half of this period, 
cytology has developed so rapidly that it has become one of the most 
important branches of modern biology. One of the landmarks in its 
progress was the appearance, at the end of 1896. of E. B. Wilson’s 
book on ‘ The Cell,’ and we look forward with great expectations to the 
new edition which, it is understood, is in an advanced stage of prepara- 
tion. A great stimulus to cytological work resulted from the rediscoverv 
in 1900 of the principle of heredity published by Mendel in 1865. which 
showed that a relatively simple conception was sufficient to explain the 
1923 K 
