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concerned with the origin and history of the germ cells from one generation to the 

 next generation," and not merely those concerned with maturation. It is, in other 

 words, a study of the continuity of the germ-plasm. The suggestion of such a 

 continuity we owe to Jager, but it was first brought into prominence by Weis- 

 mann, and it is his name that is usually associated with the idea. Broadly stated, 

 the theory maintains that the developing ovum gives rise to two series of 

 descendants, one of which forms the soma, or body, and the other the germ cells 

 of the succeeding generation, and that these two lots of cells are separated one 

 from the other from the beginning. The ideal condition would be realised if at 

 the two cells resulting from the first segmentation division should be the mother 

 cell of the body cells and the mother cell of the germ cells. Such a condition, 

 whether realised or not, has never yet been observed. The nearest approach to it 

 is found in the fly Miastor, where the first definite cell to be segmented off is the 

 primordial germ cell ; but the rest of the egg, instead of being a single cell, is a 

 syncytium with eight nuclei. Thus, although absolute morphological continuity 

 has not yet been shown, a near approach has been made both in this fly and 

 again in Ascaris, where in the 32-cell stage it is possible to pick out one cell that 

 gives rise to all and nothing but the germ cells. 



In these and other animals, a modified part of the cytoplasm or a peculiar 

 differentiation in the chromatin either within or without the nucleus is observable 

 at an early period, sometimes before fertilisation, and this differentiated part is 

 passed to the germ cells. Thus it is possible in a number of cases to follow the 

 line of descent of the primordial germ cell from the ovum, and such a line is often 

 termed the Keimbahn. The structural differentiations which seem to settle this 

 line are spoken of as the Keimbahn determinants, and both the line and deter- 

 minants are fully dealt with in this volume. 



The earlier workers on the germ cells, and particularly those who were 

 interested in the cytological phenomena underlying heredity, laid practically the 

 entire stress on the nuclei, and in particular upon the chromosomes. It is 

 undoubtedly true that these structures play a large part in heredity, e.g. the X and Y 

 chromosomes in the determination of sex, but the evidence brought together by 

 Dr. Hegner shows quite clearly that the cytoplasm of the ovum is also extremely 

 important. Indeed, the behaviour of certain inclusions in the cell, which have 

 received a multitude of names but are perhaps most generally included in the 

 term mitochondria, indicates that they influence strongly the future of the cell in 

 which they occur. As the author points out, the belief is gaining ground that the 

 phenomena of heredity are due to the interaction of nucleus and cytoplasm, and 

 not, as was formerly held, entirely to the activities of the former. 



In the last decade more and more attention has been paid to the history of the 

 germ cells during the early segmentation stages, and in consequence a large 

 literature has grown up. That this work has not been unproductive is clearly 

 shown by the present book, which aims at setting forth the most recent advances 

 in this field of research. The accounts are well and concisely written by one who 

 has himself been an active investigator of such problems, and the illustrations are 

 throughout good. The long literature list and indices add much value to a very 

 useful book. 



C. H. O'D. 



