PREFACE. V 



The first of these relates to the continuity which exists at 

 all periods of development between the protoplasm of adjacent 

 cells — a continuity which is more marked in the segmenting 

 ova and younger embryos than in the older embryos and 

 adult. It would appear indeed that in Peripatus the cells of 

 the adult, in so far as they are distinct and sharply marked off 

 structures, are not, as appears to be generally the case, present 

 in the earliest embryonic stages, but are gradually evolved as 

 development proceeds. In other words the cell theory, if it 

 implies that the adult cells are derived from embryonic cells, 

 which have been directly produced by the division of the ovicell, 

 does not apply to the embryos of Peripatus. 



The second subject concerns the ccelom. This organ is 

 more fully defined, and its condition in certain of the higher 

 groups more clearly brought out than has hitherto been done. 



Some of the figures on PI. I. are not perhaps quite as clear 

 as they should be. This is particularly the case with regard 

 to Figs. 10, 11, 12, 13. These figures are all drawn from the 

 ventral surface, i.e. from the surface opposite to the ectoderm 

 patch, so that the latter (ec.) is in all of them supposed to be 

 further removed from the observer than the endoderm mass {en.). 

 To explain them still further I may add that Fig. 16 on PI. lY. 

 represents a transverse section through an embryo of the stage 

 which these figures illustrate (near one side of the ectoderm 

 patch). Further, Fig. 8 is viewed from the animal pole : the 

 difference in level of the ectoderm patch (ec.) and the endo- 

 derm masses (en.) is not brought out in the lithograph. Fig. 

 10 on PI. III. is a diagram illustrating a transverse section 

 through this stage. 



Tkinity College, Cambridge. 

 9 August, 1888. 



