During pregnancy in the mouse the pubic bones become separated, and the gap is bridged by a ligament which at parturition may be 5 or 6 mm. long [Gardner, 1936]. The normal course of separation of the two bones in pregnant mice has been described in a previous paper [Hall & Newton, 1946a]. The bones begin to separate on the 13th day of pregnancy, and the gap widens an average of 1 mm. per day until parturition, which normally takes place during the night following the 19th day. After this the gap rapidly closes, is usually less than 2 mm. on the 3rd or 4th day post partum, but does not completely return to the virgin condition.The changes which occur during pregnancy can be produced in the spayed mouse by the administration of an extract of pregnant rabbit serum (shown by Hisaw [1926] to contain relaxin), accompanied by a dose of oestrogen which is itself ineffective [Hall & Newton, 19466, 1947]. The relaxin extract is ineffective if given without oestrogen.This paper describes the histological changes which take place at the region of the symphysis pubis in the untreated pregnant mouse, and in the ovariectomized mouse treated with relaxin and oestrone. Ruth [1937] has described the histological changes which take place in the symphysis of the pregnant guinea-pig, while Gardner [1936] compared the structure of the symphysis of virgin mice with those of the multiparous animal and of mice treated for long periods with oestrogens, but did not follow the symphysis throughout the course of the first pregnancy. MATERIAL AND METHODSAll the mice used were sexually mature albinos, and had never previously littered. In estimating the stage of pregnancy, the midnight following the finding of the seminal plug was regarded as the end of the 1st day of pregnancy. Mice were killed each day from the 11th onwards, and also at intervals following parturition in order to study the changes that take place during regression of the ligament. The pelves of intact and spayed virgin mice were also examined. Ovariectomized mice received daily subcutaneous injections of 25/xg. of oestrone dissolved in ground-nut oil, alone or + 0-2 ml. of the relaxin-containing extract of pregnant rabbit serum, the injections of oestrone being started 2 days earlier than those of relaxin. The relaxin was prepared by the method of Abramowitz, Hisaw, Kleinholz, Money, Talmage & Zarrow [1942]; fuller details of experimental procedure have been given in a previous paper [Hall & Newton, 1947]. A total quantity of 1-4-1-6 ml. of relaxin extract given during 7-8 days was sufficient to produce a pubic separation of the order of that which occurs at the end of pregnancy.

Two Boxer dogs developed progressive ataxia in association with a neoplastic infiltration of the spinal leptomeninges. In the first dog, the leptomeningeal neoplasm encompassed the entire cord and the ventral aspect of the brainstem and extended bilaterally into the piriform lobes. In the second, the neoplasm surrounded the C1-C3 segments of the spinal cord and the brainstem without involvement of the brain or spinal cord parenchyma. In both dogs, the neoplastic cells had variably distinct cell borders, clear to eosinophilic cytoplasm, and a round to ovoid hyperchromatic nucleus. Neoplastic cells were immunopositive for Olig2 and doublecortin in both dogs and for vimentin in one dog but were immunonegative for glial fibrillary acidic protein, S-100, CD34, E-cadherin, cytokeratin, CD3, and CD20. The morphological and immunohistochemical features of the neoplastic cells were consistent with an oligodendrocyte lineage. This hitherto poorly recognized neoplasm in dogs is analogous to human leptomeningeal oligodendrogliomatosis.

This paper is intended to provide a brief description of the reproductive processes of the Field Vole. Since the wtrous cycle of this species is similar to that of the Bank Vole (Brambell and Rowlands, 1936) lengthy description is unnecessary, but it seems desirable to put on record the more important results of our investigation because of the considerable economic importance of the species as a pest. It emerges, in this connection, that not only does the Field Vole appear to have a more extended breeding season (Baker and Ranson, 1933), but it also tends to produce a larger litter at a birth than the Bank Vole.Material.-The animals were obtained either by trapping or by catching them by hand in the fields when the hay was being cut. The material was collected over a period of four years (May 1931 to July 1935), and consisted of 222 separate specimens and 3 litters of nest young of Microtus agreatis hirtus (Bellamy), chiefly from Caernarvonshire and Anglesey, and 22 female specimens of M. agreatis neglectus (Thompson) from Argyll. The Scottish material was only utilized for histological purposes and as supplementary to the Welsh material.The technique, method of classification of the material, and treatment of the data were similar to those employed in the investigation of the reproduction of the Bank Vole (Brambell and Rowlands, 1936).Sex Ratio.-The material of M. agreatis hirtus consisted of 112 males and 110 females, giving a sex-ratio of 50.45f2.26 males per cent.Groutth.-One litter of nest young, which appeared to be new-born, had an average weight of 2.2 gm., which did not exceed that of the latest fetuses obtained and may be taken as the approximate weight a t birth. The lightest animal obtained, other than nest young, was 5 gm., but since few were taken between this weight and 10 gm., it may be assumed that young animals a t weaning tend to be between these weights. The lightest pregnant animal obtained weighed 12.8 gm., but the majority weigh over 19 gm. The lightest fecund male weighed 15 gm. Very few males over 20 gm. wefe non-fecund, although one was obtained which weighed 28 gm. Therefore in both males and females the weight at puberty is, as a rule, between 12 and 20 gm. The heaviest animal obtained was a pregnant female weighing 47 gm. The heaviest male was 41 gm. During the months of December and January the weights of both males and females range between 15 and 22 gm., but during February, March, and April the body-weights rise rapidly and, during summer, range up to the maximum weights recorded.Breeding Season.--Rreeding does not continue throughout the year, and during the winter months the females are in ancestrus and the males are nonfecund. The presence of mature spermatozoa in the testes, determined histologically, was adopted, as in previous work (Brambell, 1935 ;Brambell and Rowlands, 1936), as a convenient criterion of fecundity. The earliest date in spring a t which a fecund male was obtained was February 8th. The ,

Summary. The material consisted of 277 Lesser Shrews, of which 156 were males, 120 females, and one intersex (described elsewhere, 1936). The sex‐ratio of the whole sample was 56–52±2.01 males per cent. The percentage of males obtained in April and May was higher than during the rest of the year. The proportion of Lesser to Common Shrews obtained was 16±6 per cent., but in one small wood Lesser Shrews were found to be the commonest species. Young Lesser Shrews when they first appear in the traps in summer weigh 2–75 to 4 gm. They do not breed in their first season, and remain at approximately the same weight until the end of the following March, when they grow to the adult size of 4 to 6 gm., the heaviest obtained being a female weighing 7.3 gm. Adult animals die at the end of the breeding‐season. The breeding‐season in North Wales begins in mid‐April, reaches its height in June, and ends in October. Since no non‐pregnant females with recent corpora lutea in the ovaries were obtained, ovulation appears to result in pregnancy in 100 per cent, of cases. The mean number of follicles ovulated at œstrus is 6.8, and the mean number of implanted embryos in utero is 6.2. The litter size is therefore 0.5 smaller than in the Common Shrew. The œstrous cycle closely resembles that of the Common Shrew. The first œstrus is preceded by prolonged vaginal cornification and gradual hypertrophy of the reproductive organs. The largest follicles observed were 328 μ. in diameter. There is a post‐partum œstrus at which the majority of females become pregnant, gestating and lactating simultaneously. Animals that do not become pregnant at the post‐partum œstrus exhibit an anœstrous period during lactation. Forty‐seven pregnant females were obtained, 4 with tubal ova, 12 with free uterine blastocysts, and 31 with implanted embryos. Migration of blastocysts from one uterine cornu to the other takes place freely, as in the Common Shrew. The histological changes in the uterus and vagina during pregnancy closely resemble those in the Common Shrew. The data regarding the relation of the weight of the testes to body‐weight are given, and the relative sizes of the various male reproductive organs are compared with those of the Common Shrew. The authors' thanks are due to Dr. A. S. Parkes, F.R.S., and Mrs. Parkes, for their advice and for collecting part of the material in Kent, and to Messrs. L. H. Jackson and I. W. Rowlands for assistance in the routine trapping in North Wales. The authors are indebted to the Rt. Hon. Lord Penrhyn for permission to trap on his estates, where many of the animals were obtained. The expenses of this research were defrayed in part by grants from the Government Grant Committee of the Royal Society to one of us (F. W. R. B.), for which we wish to express our thanks.