PREFACE. 



THE present Monograph is a reprint of a series of papers 

 published in the Journal of Anatomy and Physiology during the 

 -years 1876, 1877 and 1878. The successive parts were struck 

 off as they appeared, so that the earlier pages of the work were 

 in print fully two years ago. I trust the reader will find in this 

 fact a sufficient excuse for a certain want of coherence, which is 

 I fear observable, as well as for the omission of references to 

 several recent publications. The first and second chapters 

 would not have appeared in their present form had I been 

 acquainted, at the time of writing them, with the researches 

 which have since been published, on the behaviour of the ger- 

 minal vesicle and on the division of nuclei. I may also call 

 attention to the valuable papers of Prof. His 1 on the formation 

 of the layers in Elasmobranchs, and of Prof. Kowalevsky 2 on 

 the development of Amphioxus, to both of which I would 

 certainly have referred, had it been possible for me to do so. 



Professor His deals mainly with the subjects treated of in 

 Chapter III,, and gives a description very similar to my own of 

 the early stages of development. 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 forward 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 con- 

 firmation of the views expressed in chapter VI. on the develop- 



1 Zeitschrift f. Anat. u. Entwicklungsgeschichte, Bd. II. 



2 Archivf. Micr. Anat. Bd. xm. 



