32 DEVELOPMENT AND GROWTH OF 



in addition to these, the formative cells (as has been shewn by 

 Peremeschko, Oellacher, and Klein, whose observations I can 

 confirm) begin to travel towards the circumference, and to pass 

 in between the epiblast and hypoblast. 



Both the formative cells, and the lower layer cells enclosed 

 between the hypoblast and epiblast, contribute towards the 

 mesoblast, but the mode in which the mesoblast is formed is 

 very different from that in which the hypoblast originates. 



It is in this difference of formation that the true distinction be- 

 tween the mesoblast and hypoblast is to be looked for, rather than 

 in the original difference of the cells from which they are derived. 



The cells of the mesoblast are formed by a process which 

 seems to be a kind of free cell formation. The whole of the 

 interior of each of the formative cells, and of the other cells 

 which are enclosed between the epiblast and the hypoblast, 

 become converted into new cells. These are the cells of the 

 mesoblast. I have not been able perfectly to satisfy myself 

 as to the exact manner in which this takes place, but I am 

 inclined to think that some or all of the spherules which are 

 contained in the original cells develop into nuclei for the new 

 cells, the protoplasm of the new cells being formed from that 

 of the original cells. 



The stages of formation of the mesoblast cells are shewn 

 in the section (PI. I, fig. 2), taken from the periphery of a 

 blastoderm of eight hours. 



The first formation of the mesoblast cells takes place in 

 the centre of the blastoderm, and the mass of cells so formed 

 produces the opaque line known as the primitive streak. This 

 is shown in PI. I, fig. 9. 



One statement I have made in the above description in 

 reference to the origin of the mesoblast cells, viz. that they are 

 only partly derived from the formative cells at the bottom 

 of the segmentation cavity, is to a certain extent opposed to 

 the statements of the three investigators above mentioned. 

 They state that the mesoblast is entirely derived from the 

 formative cells. It is not a point to which I attach much im- 

 portance, considering that I can detect no difference between 

 these cells and any other cells of the original lower layer except 

 that of size ; and even this difference is probably to be explained 



