PEEFACE. 



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 earher 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 publica- 

 tions. 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 

 germinal vesicle and on the division of nuclei. I may 

 also call attention to the valuable papers of Prof. His^ 

 on the formation of the layers in Elasmobranchs, and 

 of Prof Kowalevsky^ on the development of Am- 

 phioxus, 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 develop- 



^ Zeitsckriftf. Anat. u. Entwicklungsgeschichte, Bd. n. 

 2 Archivf. Micr. Anat. Bd. xiii. 



