IV PREFACE. 



ment. His interpretations of the observed changes 

 are, however, very different from those at which I 

 have arrived. Although this is not the place for 

 a discussion of Prof His's views, I may perhaps state 

 that, in spite of the arguments he has brought for- 

 ward in support of his position, I am still inclined 

 to maintain the accuracy of my original account. The 

 very striking paper on Amphioxus by Kowalevsky 

 (the substance of which I understand to have been 

 published in Russia at an earlier period) contains 

 a confirmation of the views expressed in Chapter VI. 

 on the development of the mesoblast, and must be 

 regarded as affording a conclusive demonstration, that 

 in the case of Vertebrata the mesoblast has primi- 

 tively the form of a pair of diverticula from the walls 

 of the arch enter on. 



The present Memoir, while differing essentially in 

 scope and object from the two important treatises 

 by Professors His^ and Gotte^, which have recently 

 appeared in Germany, has this much in common with 

 them, that it deals monographically with the develop- 

 ment of a single type : but here the resemblance ends. 

 Both of these authors seek to establish, by a careful 

 investigation of the development of a single species, 

 the general plan of development of Vertebrates in 

 general, if not of the whole animal kingdom. Both 

 reject the theory of descent, as propounded by Mr 

 Darwin, and offer completely fresh explanations of the 

 phenomena of Embryology. Accepting, as I do, the 

 principle of natural selection, I have had before me, in 

 writing the Monograph, no such ambitious aim as the 



^ Erste Anlage des Wirbelthierleibes. 

 2 Entwicklungsgeschichte dcr Unke. 



