36 PROTOPLASM 



stands out in successfully manufactured drops might well 

 awaken the supposition that one is dealing with a skin-like 

 hard covering, yet more accurate investigation shows its 

 complete fluidity. It is quite astonishing to see how easily 

 such drops, together with their border, flow along, and how 

 the latter, in spite of its fluid nature, maintains itself 

 throughout. This again just shows that it owes its origin 

 to physical laws, which continue to act during the move- 

 ment of flow. 



It is at once readily understood that the same conditions 

 which give rise to an alveolar layer at the surface of the 

 froths must bring about the formation of a similar radiate 

 layer round each of the more considerable vacuoles in their 

 interior. This is also entirely confirmed by accurate in- 

 vestigation of the froths (see Photogr. I.-III. and V.) Every- 

 where one notices round the vacuoles the finely striated 

 border, depending on the radiate arrangement of the lamellae 

 arranged closely round the wall of the vacuole. As a rule, 

 I only found ' a distinction between the external alveolar 

 layer and this border to the vacuoles to the extent that 

 the latter was usually not so sharply and distinctly marked 

 off from the neighbouring irregular alveolar substance as was 

 generally the case with the external alveolar layer. 



It was mentioned above that the alveolar layer is 

 marked off from the external fluid by a sharp, dark border, 

 which appears like a delicate darker skin or pellicle. Since 

 ordinary drops of oil also appear, when sharply focussed, to 

 be limited by a corresponding delicate dark border, I am 

 convinced that this border limiting the alveolar layer is no 

 special structural feature, but only an optical phenomenon. 



In the account given of the alveolar layer, emphasis was 

 laid on its formation out of very small alveoli quite regular 

 in size. In general it is a striking fact in the in- 

 vestigation of the froth-drops that their peripheral, and 

 therefore, of course, their entire superficial region, consists, as 

 a rule, only of very minute or small alveoli. Larger ones, 

 up finally to those so considerable in size that they merit 

 the denomination of vacuoles, make their first appearance at 

 some depth below the surface. This phenomenon shows up 



