Mat U, 1883. 



SCIENCE. 



393 



ing to polar globules, thrown off from the germ-cells 

 before they develop into embryos; third, the ' residual 

 nuclei ' of the germogens set free, as the final event 

 in the history of infusorigens. The infusorigen is a 

 group of cells, consisting, at one period, of a periph- 

 eral layer of cells partially enveloping a large central 

 cell. Its development from a single cell by a process 

 of cleavage, and the epibolic growth of its periph- 

 eral layer, give ground for believing that it passes 

 through a gastrula stage. In diphygenic individuals 

 the germ-cells are different for the two kinds of em- 

 bryos. The first to appear, one or two at a time, are 

 the large germ-cells of the infusoriform embryos. 

 After these embryos escape, there remain in the 

 parent-body small cells, which multiply until they 

 fill the greater portion of the axial cell, and eventu- 

 ally give rise to vermiform embryos. The difference 

 between developmental division (cleavage) and multi- 

 plicative division of cells is here brought in striking 

 contrast. No definite evidence of fecundation has 

 been obtained, but it perhaps occurs with one form 

 of embryo. In the development of the vermiform 

 embryo, karyokinesis splits the germ-cell into two 

 unequal parts. Then follows a three, and next a 

 four celled stage, in which three cells form a cap 

 over the fourth. This leads to a gastrula, in which a 

 single entoderm-cell is enveloped by a small number 

 of ectoderm-cells. The blastopore closes, and the 

 multiplication of cells at this pole soon leads to 

 the pyriform embryo, of which the pointed end is the 

 blastoporal region; while the rounded end corre- 

 sponds to the future cephalic pole. In this stage the 

 first germ-cell appears at the hind end of the ento- 

 derm; the second germ-cell, at the anterior end; and 

 from these two arise the other germ-cells. There is, 

 therefore, a triploblastic stage, if we regard the two 

 germ-cells as representing the mesoderm. 



It may be added, that important errors of van Bene- 

 den have been corrected by Whitman, whose article 

 is one of unusual interest and merit. As to the 

 relationship of the dicyemids, Whitman says, "I see 

 no good reason for doubting the general opinion 

 that they are plathelminths, degraded by parasitism. 

 Whether they, and their allies the Orthonectidae, 

 have descended from ancestors represented now by 

 such forms as Dinophilus, or from the Trematoda, 

 is a question which further investigations must de- 

 cide." C. S. MiNOT. 



TEMPERA TURE AND ICE OF THE BA VA- 

 RIAN LAKES. 



After an account of temperature observations on 

 Swiss lakes by earlier observers, as Brunner and 

 Fischer, Simony and Forel, A. Geistbeck (Ausland, 

 1882, 961, 1006) gives a detailed tabulated statement 

 of his observations during 1881 on sixteen Bavarian 

 lakes, showing the following results. As to varia- 

 tion with deptb, the first six metres are almost con- 

 stant; between six and eighteen metres there is a 

 rapid cooling; then, to fifty metres, a slow decrease; 

 and, below this, an almost constant temperature of 

 a little less than 5° C. Daily variation is distinct 

 to six metres, but ends at eighteen. Annual varia- 

 tion is reduced to from 0.2° to 0.9° at the bottom 

 of the deeper lakes. Two groups are noted. The 

 warm lakes, with an average temperature of 7..3° to 

 17°, are less than one hundred metres deep, their 

 bottom temperature is below 5°, and they have a 

 decided annual variation through their entire depth. 

 The cold lakes, Konigs, Starnberger, Walchen, and 

 Achen, are from 115 to 196 met. deep, and, below 



fifty metres, are always cooler than 5°, with an aver- 

 age temperature of 5.2° to 5.6°: these have, there- 

 fore, a great volume of cold water even in midsum- 

 mer, and a slow and small annual temperature range. 

 The cause of this difference is seen partly in the 

 depth of the lakes, and further in the relation of lake- 

 surface to drainage-area, which, in the cold lakes, 

 averages 1 to 10, and, in the warm, 1 to 30. Ex- 

 ceptions, here and elsewhere, to the rule of depth, 

 are Barm (31.5 met. deep), Gosau (63), and Toplitz 

 ( 105), which belong under the cold group ; for, in spite 

 of their moderate depth, they are well protected by 

 steep shores from warming by sun and wind. On the 

 other hand, Geneva (334) and Gmundener (190) ap- 

 proach the warm group, presumably on account of 

 their large drainage-area. Certain small mountain- 

 lakes, fed mostly by springs, show a relatively low 

 summer and high winter temperature. Form of the 

 bottom, and nearness to entering-streams, have strong 

 control over the water's warmth. The lacustrine 

 flora and fauna are determined chiefly by tempera- 

 ture and light. Eeeds and algae are common along 

 shallow shores, but all rooted plants end at a depth of 

 twelve metres. The littoral molluscan and crustacean 

 fauna disappears at twenty metres. In deeper water 

 there is a special 'pelagic' fauna. (In this connec- 

 tion, see Forel, La faune pglagique des lacs d'eau 

 douce, — Arch. sc. phys. nat., viii. 1SS2, 230.) 



The lake temperatures fall quickly in the autumn 

 by circulation, but rise slowly in the spring by con- 

 duction and wave action. In winter a temperature 

 lower than that of maximum density penetrates to a 

 considerable depth : less than 3° has been found at 

 forty metres. Ice forms first on the shallows along 

 the shore, and spreads outward. The high lakes freeze 

 every year, sometimes as early as October or Septem- 

 ber; the larger lower lakes, at later dates and more 

 seldom. Walchen has frozen over only three times 

 in this century; Constance, seven times since 1277; 

 Gmundener, five times in the last four hundred years. 

 In the severe winter of 1879-80 Tegern closed on 

 Dec. 21; Zurich, in the middle of January; Walchen, 

 on Feb. 3; and Constance and Gmundener, on the 6th. 

 Changes of temperature produce long cracks in the 

 ice, so characteristic as to have local names — lehnen, 

 schube, wunen, frageln — on the different lakes. 

 Further description is given of the thickness and 

 color of the ice, and certain peculiarities in the freez- 

 ing of some of the lakes. W. M. Davis. 



LETTERS TO THE EDITOR. 



CorrespondenU are requested to be as brief as possible. The 

 laj'iter^s name is in all cases required as proof of good faith. 



Zoological 'regions,' 



My attention has been drawn, by a notice in one of 

 the last numljers of Science, to what seems to me to 

 be sources of error. I refer to the determination of 

 zoological regions by percentage calculations, and the 

 idea that regions should have a certain amount of 

 numerical equivalence. This seems to be an artifi- 

 cial and hence fallacious method of dealing with the 

 subject, engendered by the lack of a proper concep- 

 tion of the matter under consideration. No defini- 

 tion or description of a ' region,' or synonymous word, 

 can be found in any of the leading works on zooge- 

 ography; but, if we put two and two together, an idea 

 can be formed which will, I hope, help solve some 

 mooted questions. 



Regions are known to differ in the kinds of animals 

 occupying them, as well as in location. All, or all but 

 one, are geographically very distinct; and all are well 



