7 i2 SCIENCE PROGRESS 



on the Platyhelminthes might well have included a reference to Bresslau's papers 

 on the development of Rhabdoccela and Alloiocoela (1904) and the Acoela (i9°9)- 

 Mention might have been made of Herber's paper on Anodonta cellensis (191 3) in 

 the chapter on molluscs. It is also to be regretted that there was not time to 

 include even in footnotes the work of Schleip on two species of Clefisine a.n& von 

 Wenck on Macrobiotus, both of which appeared in the first half of 1914. The all 

 too short account of the Tardigrada is based on Erlanger's work on Macrobiotus 

 macronyx in 1895. This investigation was mainly carried out on whole mounts, 

 and the illustrations are very diagramatic. M. lacustris has been studied by von 

 Wenck, who mainly relied on sections prepared with an improved technique, and 

 who consequently has been able to give a much more satisfactory description of 

 the early stages. 



The introductory chapter is very concise, but perhaps some account might have 

 been given of the apparent organisation of the ovum in some cases even before 

 fertilisation and of the role played by its cytoplasm in determining the fate of the 

 cells derived from its different parts. This is a subject that has received much 

 attention of late years, especially from a number of American biologists. Another 

 point, connected with this, concerns the origin of the germ cells, and here we 

 think it would have been advisable to indicate that in certain cases these cells 

 can be recognised at a very early period in segmentation before definite germ 

 layers have been established. This might have been done in the first page of the 

 introduction, where two instances are cited to show the errors that may arise from 

 an examination of an incomplete series of developmental stages. In each case 

 the germ cells were first stated to arise from the ectoderm, whereas they are now 

 said to be derived from the ccelomic epithelium. One cannot help recalling that 

 even in four vertebrates it has been shown that the germ cells, although long 

 stated to be derived from the ccelomic epithelium, are in reality migrants into this 

 layer, and may be recognised before they take up their definite position. The 

 remainder of the chapter is well worth reading, and Driesch's attitude towards 

 the law of biogenetics is effectively criticised. 



Like the introduction, the summary also repays reading, although we fear that 

 the Hormone theory, enticing as it appears, has been too readily accepted and 

 strained beyond its legitimate bounds. 



In spite of the small criticisms that have been put forward above, mostly 

 matters of opinion, the book on the whole reaches and maintains a very high 

 level, and the need for such a work has long been felt by the student of biology 

 in this country. It has the outstanding merit of being concise, easily followed, 

 and readable throughout. Much interest is added to it by the incorporation of 

 the principal results of experimental work ; these are not only important in 

 themselves, but shed light on many morphological questions. The printing and 

 illustrations are of a very high quality. Indeed, Prof. MacBride is to be con- 

 gratulated on producing a book that will be indispensable to the senior student 

 and any one else who wishes to become familiar with the facts and teachings of 

 invertebrate embryology. 



C H. O'D. 



The Germ-cell Cycle in Animals. By Robert W. Hegner, Ph.D. [Pp. x + 

 346, with 84 illustrations.] (New York : The Macmillan Co., 1914. 

 Price -]s. 6d. net.) 



The title of this book perhaps does not indicate quite clearly its scope. Germ-cell 

 cycle is a term employed by the author "to indicate all those phenomena 



