The cycle of the seminiferous epithelium of the boar was divided into eight stages which are described. The relative frequencies of the stages were determined for ten Yorkshire and ten Lacombe boars approximately 11 months of age. The mean relative frequencies for stages 1 through 8 were 10.8, 14.4, 3.5, 11.6, 8.9, 20.3, 18.5 and 12.0%, respectively. Generally, there was a constancy of the relative frequencies of the stages between breeds, among boars within breeds and between testes within boars.The absolute duration of the cycle of the seminiferous epithelium and the duration of spermatozoan transit through the epididymis were determined by injecting a single dose of 19 mCi of thymidine-methyl-H3 into the marginal ear vein of each of four Yorkshire (15.2 months old) and four Lacombe boars (14.9 months old). Unilateral castrations were performed at 0.2, 11.5, 20.0, 24.0 and 25.0 days after the isotope injection. Two intact and two unilaterally castrated animals were ejaculated for eight and seven weeks, respectively, to determine the interval between the thymidine-H3 injection and the appearance of the isotope in the ejaculated semen. Autoradiographs were prepared from the testis tissues and the semen smears. The most mature germ cells that incorporated the thymidine-H3 were the late preleptotene primary spermatocytes. Twenty-five days afer injection the label had progressed from late preleptotene primary spermatocytes to spermatozoa leaving the testes. Epididymal transit times for four boars (ejaculated daily) were 9, 10, 10 and 12 days. The duration of one cycle of the seminiferous epithelium based on 12 estimates was 8.6 i 0.1 days. The durations of stages 1 through 8 were 0.9, 1.2, 0.3, 1.0, 0.8, 1.7, 1.6 and 1.0 days, respectively. The life span of primary spermatocytes was 12.3 days, of secondary spermatocytes 0.4 days, of spermatids with round nuclei 6.3 days, o f spermatids with elongated nuclei 1.5 days, and of spermatozoa 6.2 days.Within the seminiferous tubules of sexually mature animals the cells are not arranged at random but are organized in a series of well defined cellular associations or stages. These stages succeed one another in any given area with the passage of time. This orderly sequence gives rise to the socalled "cycle of the seminiferous epithelium". Leblond and Clermont ('52b) defined this cycle as the "series of changes occurring in a given area of the seminiferous epithelium between two successive appearances of the same cellular association". The cycle of the seminiferous epithelium has been divided into stages on the basis of the development of the spermatid acrosome (Leblond and Clermont, '52a, b) or on the basis of the morphology and cytology of the germ cells and their relative position within the seminiferous tubules ANAT. REC., 161: 171-186.(Curtis, '18; Roosen-Runge and Giesel, '50; Ortavant, '59). The latter approach was used in this study.The number of cycles of the seminiferous epithelium required for the completion of spermatogenesis depends on the species and on...

Summary. The cycle of the seminiferous epithelium of the rabbit was divided into eight stages, using as criteria the shape of the spermatid nucleus, the location of the spermatids and spermatozoa in regard to the basement membrane, the presence of meiotic figures and the release of spermatozoa from the lumen. The relative duration (frequency) of Stages 1 to 8 were 27-7, 13-4, 7-3, 11-0, 4-1, 15-7, 12-

The cycle of the seminiferous epithelium of the dog was divided into eight stages, using as criteria the shape of the spermatid nucleus, the location of spermatids and spermatozoa in regard to the basement membrane, the presence of meiotic figures and the release of spermatozoa from the lumen of the tubule. Based upon these criteria, a modification of the eight-stage system of classification of the cycle of the seminiferous epithelium was developed. Cell populations making up each stage are described. The relative frequencies of stages 1 through 8 were 21.9, 12.7, 2.8, 11.5, 8.3, 15.4, 13.3 and 14.0%, respectively. The duration of one cycle of the seminiferous epithelium was 13.6 days (SE -t. 0.7), as determined from cells labeled by tritiated thymidine. The absolute durations of stages 1 through 8 were 3.0, 1.7, 0.4, 1.6, 1 .l, 2.1, 1.8 and 1.9 days, respectively. The life span of primary spermatocytes was 20.9 days, of secondary spermatocytes 0.5 days, spermatids with round nuclei 10.5 days, spermatids with elongated nuclei up to the time they are released into the lumen, 10.6 days. Counts of the different types of spermatogenic cells in tubular cross sections revealed little or no germ cell degeneration during the two maturation divisions.The mitotic and meiotic divisions and associated processes by which spermatogonia are transformed into spermatids and by spermiogenesis into spermatozoa occur in an organized manner in mammals. Several germ cell generations develop simultaneously, and this leads to fixed cellular associations in the sexually mature male. 14s a result, at any given point of seminiferous epithelium a series of cellular associations or stages follow one another with passage of time. A complete series of stages up to the reappearance of the first one at a given point constitute the cycle of the seminiferous epithelium '50; Leblond and Clermont, '52; Ortavant, Courot and :Hochereau, '69).Despite intensive use of the dog for research, including studies pertaining to :reproduction, there seems to be little detailed quantitative information on spermatogenesis in this species. Bascom and Osterud ('25) studied the size and proportion of the seminiferous tubules. Malone ('18) and Minouchi ('28) described nuclear cytology and chromosomal behavior ANAT. REC., 173: 341-352.

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causes of piglet death were determined for 569 piglets that died between birth and weaning out ofa total of2388 born over the second to fourth parity in 124 yorkshire and 109 Yorkshire x Lacombe sow litters. Eight specific causes ofdeath were identified. starvation, crushing by the sow and stillbirths were the three main causes. unidentified causes and piglets euthanized largely because of sow death or injury were of secondary importance. Exposure, congenital abnormalities and disease were of minor importance. In addition, the primary underlying cause of death appears to be a lack of adequate nutrition for the piglets as only 6.3% of the piglets dying during the first 3 d had an increase in body weight and only 15.4% of the piglets dying after day 3 had body weight increases that could be considered as adequate for their age.Key words: Piglets, death, birth, lactation lCauses de la mortalit6 des porcelets de la naissance au sevrage.] Titre abr6g6: Causes de la mortalit6 des porcelets de la naissance au sevrage.Les causes de la mortalit6 ont 6t6 d6termin6es pour 569 porcelets morts entre la nais-