he therefore concludes that the pericardium is derived from 

 the mesoderm. The result to which my observations on the 

 bud-developnent of Perophcra vir idis have led me, in regard to 

 the origin of t.ie peribranchial rudiment, is in accord with 

 that of Salensky. 



At about the stage represented in Fig. 9, a very 

 loose patch of cells is found applied to the outer surface of 

 the inner vesicle higi: up on the right side in the posterior 

 end of the bud. Before this time many isolated cells are 

 seen adhering to the wall of the vesicle at numerous points, 

 Figs. 7 and 6, but when the difference in thickness between 

 the right side and the r.- st of the vesicle is just becoming 

 apparent, a marked tendency in the scattered cells to acciimu- 

 late in one spot is noticed. At first there is but a single 

 layer of cells joined loosely toget/ier end to end, and forming 

 a somewhat elongated patch; this is the rudiment of the peri- 

 cardium., v^nich is the first organ to make its appearance. In 

 Fig. 27, u, v/hich is drawn from a frontal section, an extreme- 

 ly early stage is sho'.vn. That the rudiment is formed by the 

 coming together of free mes odermal cells , I believe there is 

 no reason for doubting. At the stage represented in this 

 figure, the similarity between many of the cells scattered 

 freely about in the space between the ectoderm and endoderm, 

 and those which form this eel] -mass, is perfectly apparent. 

 There is certainly not the slightest evidence that the wall 

 of the inner vesicle evaginates, or its cells proliferate at 

 this point; the line of demarcation between the two struc- 



• 38- 



