Chromosomes were analysed in 20 Bufo arenarum embryos. The modal number found was 22 which could be arranged in 3 groups. Group I has 5 pairs of long chromosomes, Group II 4 pairs of medium-sized chromosomes, Group III 2 pairs of short chromosomes. Pairs 2, 3 and 4 were sub-metacentrics, the remainder metacentrics. The pair 7 showed a constant secondary constriction in one of its arms. No evidence of sexual dimorphism was found.
1. Heterochromatin segments, of similar size and position, appear the same, after identical treatments, in American and Japanese species. The segments differ in distribution, being entirely proximal in Japanese species, and proximal, median or distal in American species. They differ also in the amount of intercalated euchromatin. The H-patterns of T. erectum and T. kamtschaticurn are, in a few chromosomes, the same. 2. The distribution of H-segments in a species is not related to that of nucleolar organisers, but heterozygosity for H-pattern does reflect heterozygosity for the adjoining euchromatin as well. 3. The chromocentres of resting nuclei, in the absence of any fusion, correspond with the H-segments in size and number. The fusion of H-segments to form fewer chromocentres increases i) when the nuclear volume is small, ii) when the chromosome number is low, and iii) when the segments are localised near the centromeres. These factors increase the likelihood of segments being adjacent at telophase when the chromocentres are forming.
1. In both Japanese and American species of Trillium, H-segments are over-contracted through interphase and up to pachytene of meiosis. They are under-contracted throughout mitosis at low temperatures. At all other stages and in most other conditions, they are indistinguishable from euchromatin. Cold inhibited contraction of H-segments does not recover before late anaphase, even if the temperature is raised. 2. There are indications that the euchromatin can be converted to heterochromatin, and that the contraction of the euchromatin associated with H-segments is affected by the presence of heterochromatin. 3. The visible degree of allocycly (i.e. differential contraction) of particular segments differs according to the genetically determined nature of the segment, its position and length, the heterochromatin content of the whole nucleus, the extent of the cold-treatment, the fixative used and the degree of squashing. 4. The several apparent differences between the heterochromatin of American and Japanese species result from the difference in distribution of the H-segments and of the chiasmata. 5. These observations, and those of previous authors, are discussed in relation to the possible nature and function of the H-segments, and their differential contraction.
Pendant la microsporogenèse d'une plante hybride issue d'un croisement entre L. japonicus (Regel) Larsen et L. alpinus Schleich. des ponts chromatiques relient entre elles plus de 17% des cellules-mères de microspores. Ce phénomène de cytomixie pourrait être causé, au cours des divisions préméiotiques, par I'intervention de la cytocinése au moment de la division centromérique de certains chromosomes à cycle mitotique déphasé, des substances chromatiques émigrant ainsi d'une cellule à l'autre à la faveur d'une formation incomplète des parois cellulaires.
1. Cytological examinations of 417 males of Pteronemobius taprobanensis and two males of Loxoblemmus sp. have been made. 2. The spermatogonial chromosome number in all the individuals of P. taprobanensis is 15 although this species is chromosomally polymorphic with respect to chromosomes No. 4 and 6 in order of size (vide infra). 3. The behaviour of the chromosomes in homozygous individuals of P. taprobanensis is regular and of orthodox pattern. There is some loss of chiasmata from diplotene to diakinesis stage. First meiotic metaphase chromosomes consist of 7 bivalents which could be numbered as 1-7 in order of shape and size, and the univalent X-chromosome. First two bivalents almost always are ring-shaped while bivalents No. 4 and 6 appear to be heteromorphic in heterozygous individuals. 4. In heterozygous individuals of P. taprobanensis, at metaphase I, bivalents No. 4 and 6 appear as hook-shaped structure formed of a J-shaped and a rod-shaped chromosomes. The behaviour of the heteromorphic bivalents before metaphase I could be followed occasionally. Out of 417 males examined, 162 individuals are heterozygous for chromosome 4, 69 for chromosome 6 and 78 both for the chromosomes 4 and 6. 5. The origin of the chromosomal polymorphism of P. taprobanensis has been discussed. Further, since 309 out of 417 males examined are heterozygous (about 74%), the heterozygotes are likely to enjoy a selective advantage and the species is under genetical fixation. 6. The diploid chromosome number of Loxoblemmus sp. is 13. A comparison of the caryotypes of this species with that of L. arietulus has been made. The chromosomes of Loxoblemmus sp. is different from other species firstly for the relatively small size of the X-chromosome and secondly for the number of V- and rod-shaped autosomes.
Fixed tissues from 13 armadillos and cell cultures from two were examined for the presence of sex chromatin. Barr bodies were easily recognizable in smooth and cardiac muscle tissue of adult animals and in loose connective tissue of the embryos. Nervous tissue proved unsuitable for sexing both in young and adult armadillos. Cultured cells in vitro were also examined and showed prominent nuclear sex chromatin in the female animals. These results are in accordance with Benirschke's earlier findings in these animals.
The behavior of mitochondria was studied in pollen mother cells of Tradiscantia reflexa fixed with OsO4 or KMnO4 under the electron microscope. Mitochondria were found scattered throughout the cytoplasm in cells in prophase I. But with the break down of the nuclear membrane (prometaphase I), mitochondria were found intruded into the nuclear or spindle area. Besides mitochondria, dictyosomes, lipid granules and even plastids were seen in the spindle. Mitochondria in the spindle area or phragmoplast gradually increased in number with the progress of meiotic stages from metaphase I to early telophase I. In mid-telophase I, most of the mitochondria are aggregated about the equator of the phragmoplast. Vesicles were found among the aggregated mitochondria arranged in a series forming the beginning of the cell plate (late telophase I). By the cell plate formed, the mitochondria are distributed evenly between the daughter cells. The relation between the increase in elements of the endoplasmic reticulum at the boundary of the spindle followed by the break down of the nuclear membrane and the intrusion of mitochondria into the spindle is discussed.
The relative proportions of adrenaline (A) and noradrenaline (NA) containing celluler areas have been planimetrically estimated in the twelve avian species representing several orders and families. The study reveals a great variability in percentages of A and NA-containing cells in the suprarenal medulla of these species. Three possibilities have been forwarded to explain this hormonal diversity. These are. i) methylation of medullary hormones is dependent on the cortico-medullary ratio, ii) relative abundance of A and NA in a species is determined by the degree of aggression and predation behavior manifested by it and iii) differing association of A and NA in this class of vertebrates may bear some relation to the suggested avian phylogeny. Amongst these probabilities, the last one appears to fit in rather closely to our data. This assumes an efficacious accomplishment or hormonal methylation in the course of avian evolutionary ascent. The present report also indicates that the chromaffin tissue of birds under investigation is rich in acetal phosphatides, metachromatic substances and acid phosphatase. Two species (H. smyrnensis and A. phoenicurus), however, fail to reveal any trace of medullary metachromatic materials. This as well as the concept of ‘basic cytochemical unit’ in avian adrenal medulla have been pointed out and briefly discussed.