VI PBEFACE. 



The Tunicates, more than any other group, seem of recent 

 years to have occupied the attention of embryologists, and the 

 large amount of work which has been done on this group, 

 especially in France, with regard to both the sexual and the 

 asexual methods of reproduction, will be gathered from the 

 additional literature appended to Chapter xxxv., only a small 

 proportion of which could be referred to in the footnotes. 



In the Mollusca also a great deal of work has been done,, 

 especially in connection with cell-lineage, and the formation of 

 the mesoderm and of the larval kidney, in spite of which the 

 last two points still remain obscure. Since I am more familiar 

 with the Mollusca than with any other group of Invertebrata, 

 I have revised the chapters dealing with this phylum some- 

 what more thoroughly than the rest of the volume ; I have 

 appended numerous notes, inserted some fresh paragraphs, and 

 made certain alterations in the text which appeared justified 

 by recent investigations. 



I must again express regret that so long a time has inter- 

 vened between the publication of the German edition of this 

 work and the appearance of the last volume of the English 

 translation. Volumes ii., iii. and iv. came into the hands of 

 the translator only in 1897, and the task of bringing them out 

 being necessarily somewhat lengthy, it has been impossible 

 sooner to offer the completed work to the English-speaking- 

 student, to whom it should be of <rreat service. 



MARTIN F. WOODWARD. 



Koyal College of Science, London, 

 ■Iitn<\ 1900. 



