HISTOLOGY: MUSCULAR SYSTEM 381 



Throughout the animal, the muscle tissue shows uniformity in its fine structure, although there are 

 marked regional differences in fibre density, and in the degree of flattening of the layers. The flattened 

 regions are particularly easy to examine in whole- strip preparations. 



For showing the nuclei and cell membranes as well as the muscle fibres, iron haematoxylin is to be recommended; 

 to stain the nuclei against a clear background, thionin and toluidin blue are suitable; and for staining the fibres, 

 leaving the interfibrillar substance and cell membranes clear, Newton's crystal violet gives good results. The latter 

 stain is often used for chromosomes, but in coelenterate material, where muscle fibres are ubiquitously present, it is 

 not to be recommended. Thionin gives sharp and precise chromosome staining in F.W.A. and Zenker material. 

 Chromosomes can also be studied 'in negative', that is they show up as light bodies against a darkly staining nuclear 

 sap in silver preparations of Ca-formaldehyde material. 



The float musculature consists of simple undifferentiated sheets, one cell thick. There are no 

 special muscle groups. The fibres run parallel to one another in either a longitudinal or a circular 

 direction, except at certain angles where the muscle sheet is ' tailored ' into a triangular pattern, similar 

 to that described and figured for the subumbrellar muscle sheet of Forskalia swimming-bells (Schaeppi, 

 1898, p. 536). In the region of the gas-gland the saccus-ectoderm is almost devoid of muscle fibres. 



In cross-section the muscle layer can be seen to consist of a cell body layer containing the nuclei and 

 a fibre layer in contact with the mesogloea (Text-fig. 3). The cell boundaries are not usually visible in 

 sections, and are omitted from the figure. Seen in surface view (in strip-preparations) the cellular 

 outlines can be made out (PI. XXVI, fig. 4). The cells are usually five- or six-sided, the nucleus (or 

 nuclei) lying towards the centre. Binucleate cells occur in all four muscle sheets of the float, but without 

 regularity. Polyploid cells also occur, and there is reason to believe that they arise from binucleate 

 cells by metaphase combination of the chromosomes, as in certain mammalian tissues such as the liver 

 (Beams and King, 1942). The data obtained on polyploidy and the cytology of Physalia in general 

 will be presented in more detail elsewhere. The cells shown in PI. XXVI, fig. 4, with the exception 

 of a probable tetraploid on the extreme right, are diploids. 



The chromosome number most commonly encountered in diploid float-cells is twenty (PI. XXVII, 

 fig. 1). There are no constrictions visible along the length of the chromosomes which would correspond 

 to centromeres or to nucleolar organizer regions. The chromosomes are simple rods, three pairs of 

 which are particularly long (4-5 // in a typical late prophase), the remaining seven pairs grading down 

 in length (from 3-0 ju to about 1 -8 n in such a case). Because of the flattening of the layer in which they 

 lie, the chromosomes of the saccus epithelia are spread out in prophase, as if they had been treated 

 by a squash-technique. If the prophase chromosomes lie on their flat sides, that is with both chro- 

 matids in the plane of the muscle sheet, they show little lateral curvature. If, however, they lie on their 

 narrow edges, vertical to the plane of flattening, they are usually bent. Examples of both these 

 conditions are shown. None of the chromosomes is intrinsically V-shaped, although all are susceptible 

 to distortion in the flattened epithelia. 



The nuclei contain a variable number of nucleoli, as can be seen in PI. XXVI, fig. 4, but if there are 

 many of them, the individual nucleoli are small. Planimeter estimations, using camera lucida tracings 

 of 112 nuclei and their nucleoli, have been carried out, and it has been found that whatever the ploidy 

 of a cell (which may be up to 32M), the total of the surface areas of the nucleoli is between 1/12 and 

 1/19 the surface area of the nucleus in which they occur. The tissue used was the saccus-endoderm, 

 which is fairly evenly flattened over wide areas. A similar finding is reported for Ranaby Beatty (1949). 



The nuclei in the more flattened regions, particularly in the saccus-ectoderm which may be less 

 than 2 fi thick, are often so compressed within the narrow confines of the layer that they assume very 

 irregular shapes, frequently becoming elongated in the grooves between the muscle fibres. Under 

 these conditions fragmentation appears to take place. Pieces of nuclear matter become lodged on 



