364 ' LESLIE B. AREY 



forms. My reason for believing this is that I have frequently obtained 

 Planorbis, Lymnaea and Physa, in mid-winter, by breaking through 

 the ice and using a small dredge or net. 



Some believe that the fresh water pulmonates do not hibernate at 

 all. That appears to me to be based on negative evidence, since they 

 have not l)een observed under natural conditions, or rather have been 

 observed under unnatural conditions (vivaria). 



At the conclusion of all experiments, to be described subse- 

 quently, the animals were beheaded, fixation of the head and 

 the contained eyes occurring under conditions of light and tem- 

 perature identical with those at which each experiment had 

 been conducted. Perenyi's fixing fluid gave very satisfactory 

 preservation. Sections of 8 m thickness were cut parallel to 

 the long axis of the eye and were stained with Heidenhain's 

 iron-haematoxylin and with orange-G. 



Whenever the experimentation involved the employment 

 of light, strongly diffused daylight, such as is obtainable at 

 north windows, was used. 



The microscopical preparations were studied and measure- 

 ments were made at a magnification of 1400 diameters. In 

 expressing the extent of pigment distribution in nimierical 

 terms, two measurements were used. One, which I shall call 

 the 'zonal measurement,' represents the breadth of the band 

 of dense pigment, exclusive of the finger-like processes extend- 

 ing toward the bases of the constituent cells (figs. A, 4 and 5). 

 The second, or 'process measurement,' consists of the zonal 

 measurement plus the average length of the pigmented proc- 

 esses just referred to. 



The sense in which a few descriptive terms will be consistently 

 employed needs explanation. 'Peripheral' and 'central' will 

 designate those parts of the eye which are respectively farthest 

 from and nearest to its center (fig. B). 'Distal' and 'proximal,' 

 used in describing retinal cells, are retained in a sense similar 

 to that employed in descriptions of uninvaginated ectodermal 

 cells — hence distal indicates the portion nearest the lens or 

 the lumen of the optic sac, while proximal refers to that part 

 of a cell nearer the connective-tissue sheath at the periphery 

 of the eye (figs. B and C). 



