BY WILLIAM A. HASWELL, M.A., D.SC. 587 



its close connection with the epiblast has the appcaranee, 

 especially after the pi'imitive groove has become formed, of being 

 an outgrowth from an involution of the epiblast. The mesoblastic 

 plates are formed by outgrowth from this primitive mesoblast of 

 the primitive streak extending outwards between the epiblast and 

 the hypoblast. The union of the primitive mesoblast with the 

 epiblast in the axial line of the primitive streak being complete, it 

 is very difficult to say that the epiblast has no share in the growth 

 of the lateral plates ; probably the union exerts some influence on 

 the activity of the primitive mesoblast cells ; but I think we may 

 safely say in view of the facts adduced above, that the foundation 

 of the mesoblast of the whole embryo is laid by the cells of the 

 lower layer, and that no part of it up to this point is formed 

 directly from the epiblast. 



The above account of the formation of the mesoblast in the 

 emu is in substance the same, so far as I can ascertain at second- 

 hand from Keller's summary*, as that put forward by His 

 for the fowl. It is the view also maintained by Rauberf, 

 by Dissej, and by Duval §. On the other hand Kolliker|| 

 regards the mesoblast as formed by ingrowth from the epiblast 

 along the line of the primitive streak. Gerlach H also describes 

 the mesoblast as of epiblastic origin, and also Roller, who, 

 however, regards the participation of the hypoblast as probable, 

 but not certainly ascertained. Balfour** maintains that part of 

 the mesoblast of the primitive-streak region is derived from the 

 epiblast. 



In connection with this subject it has to be noted that the chick 

 as described by Balfour and others, difters from the emu, in that 



