REVIEWS 515 



than the rule for this "law" to hold, and this is reflected in the little use 

 embryology has been in the manufacture of plant phylogenies. 



It is a pity that the remarks on Xenia, on p. 74, should have crept into the 

 book. The explanation of this phenomenon, based on double fertilisation, is so 

 convincing and well-known that the explanation of "the heretofore puzzling 

 phenomena of the Xenia" put forward by the author appears very curious. 



W. S. 



Textbook of Embryology. Vol. II, Vertebrata with the Exception of Mam- 

 malia. By J. Graham Kerr, Regius Professor of Zoology in the Univer- 

 sity of Glasgow. [Pp. ix + 591, with 254 illustrations.] (Macmillan & Co., 

 1919. Price 31s. 6d. net.) 



This is the second volume of this " Textbook," following the first by Prof. MacBride. 

 In the first place we must say immediately that this is undoubtedly a great work, 

 and a worthy mate to the previous volume, and this opinion is forced on one even 

 though there may be parts of the volume with which one is obliged to find fault. 

 The Introduction is poor and skimpy ; we do not consider that such a book should 

 have been written without a better and fuller treatment of vertebrate gameto- 

 genesis. The first thirty pages are taken up with the description of various forms 

 of segmentation in Vertebrata, then follow some pages on gastrulation. The 

 author concludes that the primitive streak of birds represents the line of coal- 

 escence of the gastrular lips just as it actually is in reptiles, and that the neuren- 

 teric canal which transverses the primitive streak of the tern, duck, goose, etc., 

 represents a persisting portion of the once slit-like gastrular mouth which is other- 

 wise obliterated. 



The origin of the mesoderm is then discussed. Prof. Kerr gives what we 

 consider a very clear account of the subject. We cannot, however, agree with 

 him when he claims that the region of continuity of the mesoderm with the endo- 

 derm in forms such as Rana is ventral, and not dorsal, as Hertwig and most other 

 embryologists believe. The question is one of some importance, and the reviewer 

 cannot see how Prof. Kerr can uphold his tenets if he compares his own figure 

 34A with his figure 33A. The chapter on skin and its derivatives is interesting, 

 but surely more space might have been given to the development of the nervous 

 system? The treatment of the subject of the alimentary canal is nicely balanced. 



With reference to the renal organs, the author has a peculiar modified view, 

 which seems to us very perverse. Most workers agree that Goodrich showed 

 clearly that in Annelids the nephridial tube and ccelomic funnel were at first 

 distinct and separate structures with separate external openings, and that in 

 certain cases subsequently a more or less intimate interconnexion between the 

 funnel and the tube came about. Prof. Kerr considers that in all probability 

 the funnel and tube were originally connected, and later on tended to separate 

 from one another. We consider that the author does not fully appreciate the 

 evidence adduced by Goodrich. The subject of the migration of germ-cells in 

 early development has been enriched by the splendid work of John Beard, Allen, 

 etc., and one might have expected some sort of proper treatment of the view of 

 the early segregation of the germ-cells. What we do find is that Prof. Kerr has 

 merely stated that the " evidence (for such views) has not as yet, in the present 

 writer's opinion, reached a stage of being convincing." While we ourselves agree 

 with this statement fully, we consider something more might have been given in 

 this volume. Space does not allow us to comment much further on the Professor's 



