784 SUMMARY OF CURRENT RESEARCHES RELATING TO 



make their appearance in different places, and disturb the uniformity 

 -without justifying the conclusion that the separation is complete. 



On examining fragments of tlie shell and hoop, it is found that 

 both have round pits ; but whilst the pits on the shell are compara- 

 tively large, those on the hoop are dec'deiUy small, and this enables 

 •as to distinguish to which a given portion belongs. Eound the 

 shell, however, runs an annular zone, which is quite free fi'om pits, 

 viz. the zone of the cushion-like rim. On the halves of the hoop 

 there is, in addition to the groove, a zone without pits, which is very 

 narrow and lies near the free edge, but there are besides semicircular 

 free places, situate close to the shell-margin of the hoop ; the whole 

 remaining surface is thickly dotted. 



By means of well-corrected immersion lenses, especially with the 

 new homogeneous-immersion lenses, and by carefully using the 

 micrometer screw, it is possible to bring into focus the outermost 

 surface of the cell in such a way that the different images of the dots 

 on the hoop and the shell are recognized as immediately over one 

 another; that is, they appear one after the other. This optical reaction 

 is efiected with perfect certainty, and, with proper optical instruments 

 and manipulation, can be demonstrated to any one. 



The young shell — and this deserves particular attention — begins 

 to develope the halves of the hoop only when the cell is ready to 

 divide. When this is effected, the two cells formed remain united 

 in T. musica for some time, and apparently only separate before the 

 fresh division ; hence in this species only unicellular or double 

 individuals were met with, never any with three or four cells 

 connected. In those consisting of two cells, therefore, the two 

 young (interior) shells were always found without hoops. 



Viewed from the broad side of the hoop, when the focal adjust- 

 ment is highest, the image of the fine pits of the hoop is obtained on 

 a double cell in two places ; on lowering the tube the coarser pits or 

 the broad undotted zone come into view. At two other places, on 

 the other hand, of the same double cell, there is nothing of the kind 

 visible ; over the undotted shell there appear no hoop-pits. If we 

 define these places with reference to their position and regularity, 

 the result arrived at is that this image of the superposition can only 

 be obtained from young shells, which, according to the Macdonald- 

 Pfitzer theory, must be the enclosed ones. Denoting the enclosed 

 shells by e, the free by /, the sequence in double cells is / e e f. 



This, however, would only prove that the young shells are covered 

 by a hoop-membrane ; which would also be the case if a simple piece 

 of hoop connected both mother-cells. But in T. americana triple 

 cells are often met with, and from them a proof may be obtained. 

 The development of these triple cells is undoubtedly this : — one of 

 the double cells remains barren, whilst the second proceeds to a fresh 

 division, and at the same time remains connected with the first ; the 

 frequent occurrence of double cells in which one shows a broadened 

 hoop-space would alone prove this ; but the sequence of the free and 

 enclosed shells, which in all the cases examined uniformly corre- 

 sponded with the formula /e/e e/, puts it beyond doubt. If the 



