THE PRIMITIVE STREAK OP THE RABBIT. 209 



continues active till the formation of the extreme end of the 

 tail is completed. 



Is it possible to determine approximately which parts of the 

 embryo are formed by the two centres of growth respectively? 

 Evidence may, I think, be brought to show that the primitive 

 streak in the rabbit is the growing point of the whole of that 

 part of the animal which is situate posterior to the head, while 

 from the primary centre of activity is formed the part of the 

 embryo anterior to the first protovertebra. In other words, the 

 secondary centre of growth is responsible for the metamerically 

 segmented part of the animal. 



During the later stages of development the line of demarca- 

 tion between the two — which perhaps is never absolutely 

 definable — becomes less and less distinct. For instance, the 

 heart, which is formed distinctly in the region of the primary 

 centre of growth, becomes located more posteriorly ; while 

 parts of the nervous system, owing to the more rapid growth 

 of the neural tube, become moved forwards, so as to bring into 

 the primary region portions whose origin has been due to the 

 secondary area of activity. 



If we regard the secondary area of proliferation or primitive 

 streak simply in its functional capacity, neglecting for the 

 moment all preconceived ideas of its morphological or recapi- 

 tulatory meaning, and consider it only as we find it in the 

 rabbit embryo, it seems to me that we are bound to describe 

 it as the visible expression of an area of intense protoplasmic 

 activity, which is continuously budding ofi^ cells, the most con- 

 spicuous of which are those from its lower surface, the primi- 

 tive streak mesoblast cells. These cells, according to my 

 hypothesis, are rapidly removed from this area, being carried 

 away by the expanding wall of the blastodermic vesicle, to 

 which they are closely approximated. This part of the wall is 

 enabled to respond to the expanding influence of the hydro- 

 static pressure within, by itself receiving rapid additions of 

 cells from the proliferating area. 



On this hypothesis we may take the outline of the primitive 

 streak mesoblast as indicating also the outline of epiblast 



VOL. 37, PART 2. — NEW SEE. 



