Cytology, with special reference to the metazoan nucleus, by W. E. Agar.

Abstract

In general a cell is composed of two principal morphological constituents, the nucleus and the surrounding protoplasm, or better, cytoplasm, since some authors use the former word to include both cytoplasm and nucleus. This division of the cell constituents is quite sharp in all but the lowest organisms, the nucleus being delimited from the cytoplasm by a membrane except during a certain period of its division processes. In certain unicellular organisms (Protista), however, the word " nucleus " is inappropriate, since the material which composes this structure in the higher organisms is scattered through the cytoplasm as minute granules, or chromidia (see Chapter VI.). In other Protista both nucleus proper and chromidia are present (e.g. Diffltigia), while certain Bacteria are said to consist entirely of " nucleus." Even in multicellular organisms, both in Metazoa and Metaphyta, there are frequently minute bodies in the cytoplasm which are supposed by some cytologists to be derived from the nucleus, and to consist of true nuclear material, and therefore to be comparable to chromidia. A. THE CYTOPLASM The cytoplasm surrounding the nucleus includes a number of different structures of which the following are the most important : (i) The cytoplasm proper. (2) The cell membrane. (3) The centrosome. (4) Chondriosomes. (5) Metaplastic bodies. (i) The Cytoplasm Proper The living cytoplasm itself consists of a viscid, nearly transj:>arent substance, which often is clearly not homogeneous. Great difference of opinion exists as to its structure, chiefly owing to the fact that very Httlc I B nsnitn library N. C. State College attached to the spindle fibres. E, metaphase ; F, anaphase ; G, telophase ; H, nuclear and cell division complete, and daughter nuclei reconstituted. I MITOSIS 9 condensation ultimately form the relatively short and thick chromosomes of the later stages. Karyosomes, if present, since they are composed of chromatin, disappear, being used up with the rest of the chromatin in the formation of the chromosomes. If plasmosomes are present, they disappear either before or after the disappearance of the nuclear membrane (see below), apparently without participating in the formation of the chromosomes or playing any further part in the hfe-history of the nucleus. For some time after the thread formation, which starts in the early prophase, has proceeded or even been completed (by conversion of the entire chromatin content of the nucleus into filaments), the length of the threads is far greater than the circumference of the nucleus (Figs. 3, 6, 7, 8), and hence the nucleus is filled with a complicated tanglethe spireme of Flemming-in which it is impossible to discern how many separate filaments are present.. Numbers in thick type refer to pages on which figures will be found

Australian genetics: A brief history

“…For some time he was at the forefront of the field of cytology and carried out noteworthy research on the cytogenetics of lungfish. He also published a popular monograph, Cytology (Agar, 1920). Agar came to Australia in 1919 and became foundation Professor of Zoology at the University of Melbourne.…”

Optische Sensoren für die Bioprozeß‐Kontrolle

“…Off-line-Analysen liefern in der Regel erst nach langerer Analysenzeit rnit entsprechender Verzogerung Informationen uber den Zustand des Systems, stellen aber eine wichtige Kontrolle der kontinuierlich arbeitenden Systeme dar. Ein wichtiges Beispiel fur optische Off-line-Analysen ist die Laserdurch-flul3-Cytometrie [85]. Die kontinuierliche Analyse ist grundsatzlich nach folgenden drei Prinzipien moglich: 1) in situ, d.h. die Analyse erfolgt im Bioreaktor selbst; 2) on-line, d.h. Kulturbruhe wird steril dem Bioreaktor entnommen und nach entsprechender Probenaufbereitung analysiert; 3) nicht-invasiv, d.h. zwischen MeBsystem und Kulturbruhe besteht kein physikalischer Kontakt.…”