Vi PEEFACE. 



both in the earlier and later stages of development, may occur 

 between allied genera and species, or even amongst individual 

 members of the same species. 



The present book is an attempt to fill the gap, thus indicated, 

 so far as the elements of Vertebrate Embryology are concerned. 

 In it a few selected types are alone dealt with, and to each of 

 these a separate chapter is devoted. 



In the choice of' types I have been mainly guided by the 

 following considerations. Amphioxus is taken first, partly on 

 account of its great morphological importance, and partly 

 because of the extreme simplicity of its earlier developmental 

 history, and of the clue which this affords to the more compli- 

 cated conditions obtaining in the higher vertebrates. The next 

 three chapters deal with the frog, the chick, and the rabbit 

 respectively ; these have been selected as good representatives 

 of the classes to which they belong, and as being the most easily 

 obtained and the most suitable forms for laboratory pxirposes. 

 The final chapter, and the longest in the book, is devoted to the 

 development of the Human Embryo ; this has been included on 

 account of its great intrinsic interest, and of the difficulty the 

 student experiences, owing to the scattered and comparatively 

 inaccessible nature of the original memoirs, in obtaining a 

 reliable account of the present state of our knowledge. I have 

 taken much pains to make this chapter as complete as our 

 knowledge will allow, and venture to hope that it will be found 

 useful not only by students of science and of medicine, but also 

 by those engaged in medical practice. 



I have not attempted to write a series of complete mono- 

 graphs ; my purpose has been to give consecutive and straight- 

 forward accounts which shall contain, in a form convenient for 

 reference, the main facts known to us concerning the development 

 of the animals I have selected as types. Many points of detail 

 have been purposely omitted, as have also some of the more 

 recent statements which appear to me to require confirmation. 

 Science is better served by clearly stating in what points our 

 knowledge is defective than by ignoring or evading difficulties ; 



