AUTHORS' PREFACE 



The facts that the comparative embryology of Invertebrates 

 has not had a broad and comprehensive presentation since the 

 appearance of Balfour's Treatise on Comparative Embryology, 

 and that the special literature of this subject has undergone 

 an enormous increase since that time, have forced upon 

 every one who has been concerned with questions of com- 

 parative embryology the pressing need of a more modern 

 treatment of the subject. Inasmuch as we had occasion to 

 go over a considerable part of the literature of this subject 

 during the last few years — partly for the purpose of courses 

 of lectures to be given, partly from the requirements of 

 special investigations — it was natural that the idea should 

 have occurred to us to utilize this preliminary work, and by 

 arranging the material acquired and further elaborating it 

 to issue the whole in book form — a venture which was 

 undertaken, and the first results of which have assumed the 

 form of the present part. 



Since it has beea our plan in writing the present work to 

 proceed from the special to the general, and since naturally 

 some time will elapse before the completion of the whole, 

 we have thought that we should secure the gx-atitude of the 

 reader if we published the first half of the special part at 

 once. The second half of this part, embracing the Arthro- 

 pods, Molluscs, Molluscoidea, Tunicates, and Amphioxus, 

 will appear shortly, while we hope to finish the general 

 part, and therewith the whole book, in the course of the 

 year 1890. 



Not to begin the special part of this work too abruptly, 

 we have prefaced it with a shoi*t general introduction. 



Our decision to limit the subject matter to the so-called 



