FA'J'E OF BLASTOPORE, ETC., IX CHELOXIA. ]^5 



the primitive plate on the emhryonic shield as given in Woodcnt 

 I. B. 



II. NEW OBSERVATIONS, 



In giving an account of the ontogenetic processes which I ]jro- 

 pose to describe in the present Contrilxition, it seems most convenient 

 to go first over (I) the changes as they can be observed in surface-views 

 of the embryos, and then (2) to describe the series of sections through 

 the embryos, in order to explain more in detail the nature of the 

 changes involved in these processes. 



A. THE SURFACE CHANGES. 



Before entering into the subject proper of this section I may be 

 permitted t(3 urge here the great necessity of studying surface 

 views of embryos with the utmost care in investigations like the present. 

 It seems to me that investigators, especially in vertebrate embryology 

 have a tendency to make light of o1)servations made of the surface views 

 of embryos, and, before they have exhausted what can be learned 

 from outside, have at once recourse to " microtomizing." I have 

 never realised as in the present investigation the fact that a careful study 

 of surface views not only saves a great deal of labour by giving a 

 correct notion of the relations of the parts examined but enables us to 

 make out i^oitüs wliicli u'Oidtl ahnost certainlij escape us if studied ouhj or 

 mainhj in sections. In fact, it would not be too much to say that but 

 for an exhaustive study of the surface changes, this contribution would 

 never have been Avritten. 



I found in the course of the present investigation that in most of 

 those embryos in which the yolk-|)lug has already considerably shifted 

 its position backwards (^sce iifra), it could be made out only by turning 



