PREFACE. 



THE present Monograph is a reprint of a series of papers 

 published in the Journal of A natomy 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. n. Entwicklungsgeschichte, Bd. n. 



2 Archiv f. Micr. Anat. Bd. xnr. 



