Oct. 2^,^Z^2\ NATURE 



follicles as a matter of course, and pass along the canal leading 

 from it to a primary branch of the ovarian tube, and there two and 

 sometimes three of these eggs fuse into one mass, around which 

 a shell is secreted, and which thus forms the actual egg— really 

 a threefold egg ; and from such a wonderfully formed egg only 

 one embryo develops. Unfortunately we are not told what 

 becomes of the germinal vesicles; according to the drawings they 

 seem to disappear at this stage. We know of the development 

 in the tunicate pyrosoma of five embryos from one egg, here we 

 have the converse case of one embryo developing from three eggs. 

 Siebold appears to have convinced himself that the fusion is a 

 normal thing, and not due to any pressure or osmotic action 

 taking place during the microscopical examination. The 

 structure of the ovary of Apus is figured in a plate. 



As to the other crustaceans named, which are Artemia salina and 

 Limnadia Hcnnaititi, the occurrence of parthenogenetic broods 

 is inferred from the descriptions of other writers whose works 

 are criticised at some length, and also from examination of speci- 

 mens. It seems not impossible from an observation of Zenker 

 that in Aitcntia salina parthenogenetic alternate with digenetic 

 broods. In the beginning of the year 1S51 this observer found 

 three males among one hundred females, later in July tlie same 

 pond furnished thousands of females, but not one male. 



In conclusion. Prof Siebold, whilst adopting Leuckart's term 

 " Arrenotoky," to designate the phenomenon of the partheno- 

 genetic production of male offspring, as seen in the Ilymenop- 

 tera, proposes the parallel term, " Thelytoky," for the patheno- 

 n-enetic production of female offspring as demonstrated now 

 conclusively in some Lepidoptera and Crustacea. It seems 

 to us that a tliird term should also be available for the case 

 of mixed offspring (that is of two sexes) such as " Amphotoky ; " 

 and the terms need not be limited to parthenogenetic cases. In 

 his concluding remarks, whilst repeating the expression 'of his 

 conviction that parthenogenesis will be found more and more to 

 be of frequent and fixed occurrence in various classes of animals, 

 Siebold alludes with caution to the list of cases in which par- 

 thenogenesis is stated to occur, given by Gerstaecker in Bronn's 

 " Classen und Ordnungen des Thierreichs. " Gerstaecker rightly 

 enough distinguishes cases in which parthenogenesis has been 

 observed as an accidental and rare exception, and those in which 

 it has a definitely recurring place. Siebold considers (and after 

 the gi-eat pains he has himself expended on the cases recorded 

 in this book, he is fully warranted in so doing) that many of the 

 examples put forward by Gerstaecker require a more careful 

 testing, and he offers some remarks on parthenogenesis in the 

 gall-flies, and in the silkworm moth. Finally, he alludes to 

 cases among Vertebrates in which indications of a power of 

 development in the egg, independent of the male element, have 

 been observed. The most remarkable of these is that quoted 

 by Leuckart in his work already cited, which Siebold omits 

 here, but has done justice to in the short supplementary paper 

 read at the Munich Academy since the publication of this book. 

 In 184+ Prof. Bischoff found ova in the uterus of an unimpreg- 

 nated sow, which exhibited segmentation of the yelk, some into 

 two and four, and others into sixteen and twenty divisions. 

 Other ca«es here given are as follows :--In the oviduct of a three- 

 year-old rabbit, thoroughly separated pathologically from the 

 uterus Prof. v. Hensen of Kiel found ova in various stages of 

 yelk-division, and some of their cells had even advanced into a 

 branched condition. Dr. Oellacher of Innsbruck has observed 

 stages of yelk-division in unfeitilised hen's eggs. In fishes, 

 in 1859, Agassiz observed yelk-division occurring in the eggs of 

 Gadida;, whilst yet in the ovary, and considered it to be due to 

 impregnation, even stating that he had seen certain fishes place 

 themselves in such a position as to favour this supposed intra- 

 ovarian fertilisation. Burnett has since investigated the case, and 

 concludes that the yelk -division is independent of fertilisation, 

 a supposition which is rendered in every way probable from other 

 researches on the fish egg ; but, curiously enough, ^Dr. Burnett 

 thinks these eggs should be regarded as "germs," and not as 

 "true eggs," an opinion to which Siebold, of course, is com- 

 pletely opposed, and which, in invertebrate cases, has been 

 shown to be untenable. 



Siebold does not allude to those cases of ovarian cysts found 

 occasionally in the unfertilised human female, and containing 

 hair and teeth— a phenomenon which we should be glad to see 

 further discussed and investigated, since, as far as we can re- 

 member, the origin of the contents of such cysts from irregularly 

 developing ova is probable. The eel is suggested as a possible 

 parthenogenetic vertebrate. It is a very strange fact that we are 



52s 



still ignorant of the ripe eggs and embryos as well as of the males 

 of the eel, even as in the time of Aristotle. With the following 

 words of that greatest naturalist, addressing them to those who still 

 refuse to accept the existence of Parthenogenesis, Siebold ends his 

 book:— "More belief must be given to observation than to 

 theory, and this last is only worthy of belief when leading to the 

 same result as experience.'' E. Ray Lankestek 



ON SOME NEW POINTS IN THE MOUNTING 

 OF ASTRONOMICAL TELESCOPES'^ 



THE very great inconvenience attendant upon the use of the 

 ordinary position circle of a micrometer divided on a metallic 

 limb, and the necessity of having small lamps hung on to the 

 micrometer for producing that very useful character of illumina- 

 tion of the wires known as the " dark field," has induced me to 

 introduce some modifications in this (to the observer at least) 

 very important part of an equatorial instrument. 



These modifications have already been applied with success, 

 and for the first time (as far as I am aware) to a 7-inch refracting 

 telescope now in course of erection at the Observatory of the 

 Royal Artillery Institute, Woolwich ; and I have (in consequence 

 of this success) been ordered to adapt them to the Great Equa- 

 torials now in course of construction for the Royal Observatory, 

 Edinburgh, and the Observatory of the Lord Lindsay, Aber- 

 deen, t 



The rack and pinion tube carrying the eye piece or micro- 

 meter revolves freely in the casting which forms the lower end 

 of the telescope tube, and carries a brass plate (all cast in one 

 piece), on which is cemented a flat ring of plate glass, muffed on 

 back, and in front varnished with an opaque varnish. Through 

 this varnish the divisions are cut, so that on being illumi- 

 nated from behind, the divisions appear bright upon a black 

 ground. The vernier is similarly treated, and the whole of this 

 circle, being covered with a cap, with a glazed window only 

 sufficiently large to expose the vernier and about 15° of the circle, 

 is protected from possible injury and is read most conveniently 

 through this fwindow, being illuminated by a beam of light 

 constantly directed upon it from a lamp hanging on end of the 

 declination axis, as wiU be afterwards explained. 



Between the fixed casting which forms the end of the telescope 

 tube and that which revolves in it is another metallic circle cut into 

 360 teeth on edge, and with 90 holes drilled accurately on face : 

 into the teeth on edge is geared a screw which is mounted on fixed 

 casting, one revolution of which is of course equal to an angular 

 movement of 1°. 



In the other (outer) moveable brass circle is mounted a steel 

 pin working up and down in a small cylinder; this pin, being 

 pressed down by a small spiral spring, enters into one or other of 

 the 90 holes in the intermediate circle, and thus clamps the whole 

 eye-endto the intermediate circle, in which condition a slow 

 motion is obtained by the endless screw. When it is desired to 

 move the eye-end through a large angle, the rack and pinion 

 tube is grasped by the hand, and in doing so the hand 

 almost necessarily grasps also a small steel trigger which 

 lifts the steel pin out of the hole, frees the moveable circle, and 

 allows it to be placed in any angular position. When the de- 

 sired position is approximated, and the trigger relieved, the pin 

 drops into the nearest hole, and the endless screw is then used for 

 final setting. . 



The diagram will I think explain the various matters of illu- 

 mination.J . , . ,. 



From a lamp hanging upon the end of the dechnation axis is 

 sent a beam of slightly divergent light through this axis, which 

 is hollow ; this slightly divergent beam is utilised for sbc different 

 purposes, three portions of it being reflected out in different 

 directions to illuminate portions of the declination circle, of which 

 one is for a long reader lor setting from eye-end, and the other 

 two for micrometer microscopes subdividing the lo' division of 

 circle into single I '' arc. , , , . 



None of these are shown in diagram, but the other three 

 purposes for which the light is utilised, viz., for position cu-cle, 



♦ Paoer read before the British Association at Brighton in Section A, 

 Aw 20 bv Howard Grubb, C.E., F.R.A.S. 



t°The' breech- piece and position circle of the Woolwich Equatorial were 



"^rxhe oneinal diagram showed all three illuminations, and of different 

 colours Here it has been thought better to show the daif: held by itself, 

 and the bright field and position circle illuminations m a separate diagram. 



