174 



SCIENCE. 



[N. S. Vol. XXIV. No. 606. 



In the second chapter on 'Heredity and 

 Embryonic Differentiation ' the author gives 

 a brief account of the statistical, and a more 

 extended account of the cytological studies of 

 heredity. He calls attention to the impor- 

 tance of the nuclear linin in the preservation 

 of the characteristic niunber and form of the 

 chromosomes and he concludes that the linin 

 should always be considered along with the 

 chromatin and that both linin and chromatin 

 constitute the germ-plasm. The hereditary 

 significance of the conjugation of male and 

 female chromosomes, and of their separation 

 in the reduction division first discovered by 

 the author, is discussed. In his treatment of 

 embryonic differentiation the position is taken 

 that the ability of egg fragments to give rise 

 to whole embryos, no more indicates the un- 

 differentiated character of the fragments than 

 the similar ability on the part of adult pla- 

 narians. 



The third chapter on the ' Relations of Modes 

 of Reproduction and Conjugation ' is less con- 

 vincing in its conclusions than the preceding 

 ones. The evidence presented in favor of the 

 view that fragmentation is more primitive 

 than binary fission is not convincing; and the 

 same is true of the conclusion that Geddes and 

 Thompson's theory of sex must be abandoned 

 because sexuality is ' the state of occurrence 

 of dissimilar individuals,' only one of which, 

 the egg, has the power to reproduce. Sim- 

 ilarly the conclusion that the male is morpho- 

 logically and physiologically inferior to the 

 female, while supported by many examples, 

 meets with so many exceptions as to cast 

 doubt upon its general applicability or valua 

 The statement that ' sexuality finds its expres- 

 sion and origin in extra-chromosomal differ- 

 ences ' does not accord with the recent careful 

 work of Stevens and Wilson. And the state- 

 ment that every Metazoan formed from a fer- 

 tilized egg cell is initially a hermaphrodite, 

 because formed by the fusion of male and fe- 

 male germ cells, while an animal arising from 

 an unfertilized egg is initially female, though 

 it may later become male, seems to the re- 

 viewer to confuse sex with sex cells and there- 

 by to create many needless difficulties. Finally 

 the evidence brought forward at the close of 



the chapter in favor of the view that ' eventual 

 hermaphroditism ' is a secondary condition 

 and not primitive is not wholly satisfactory, 

 for it does not necessarily follow that ' because 

 of its [separate sexes] more general occur- 

 rence it would seem to be more primitive.' 



The fourth chapter contains an extensive 

 treatment of ' Life Cycles and Polymorphism 

 of Individuals.' The author concludes that 

 ' all cases of metagenesis have originated from 

 larval development, and that asexual repro- 

 duction is limited to an immature or larval 

 stage.' Practically he makes no distinction 

 between larval development with metamor- 

 phosis and metagenesis, or in his own words, 

 ' All discontinuous larval development is meta- 

 genetic, in that the adiilt is formed asexually, 

 by budding from the larva.' This discussion 

 is of service in showing that there is no hard 

 and fast line between metagenesis and dis- 

 continuous larval development, but there 

 seems to be no good reason for changing the 

 old definition of metagenesis, viz., ' an alter- 

 nation of a sexually produced generation or 

 person with an asexually produced one,' for 

 this new, definition, even though in some cases 

 there may be no sharp line of demarcation 

 between the alternation of ' persons ' in meta- 

 genesis with the alternation of ' organs ' in 

 larval development. 



Under ' ekdytic development ' the author in- 

 cludes those cases, commonly known as fetal 

 development, in which special annexes are 

 formed, which are later lost. He considers 

 that Beard is right in classifying fetal devel- 

 opment as discontinuous, but wrong in regard- 

 ing it as a form of larval development. 



The fifth chaptej deals with ' Variations 

 and Mutations ' and represents in a brief form 

 the relations of the mutation theory of de 

 Vries to the germ-plasm theory of Weismann. 

 The tabular classification (p. 135) of the con- 

 ditions of variation and mutation, and of the 

 resulting types of variants and mutants is a 

 brief and useful summary. As to the cause 

 of atypical forms he says (p. 146) : ' Our con- 

 clusion is that variation, progressive or re- 

 gressive, and also mutation, in fact any in- 

 herited modification of the race, is instituted 

 by stimulus of the environment ' ; and again 



