April 29, 1921] 



SCIENCE 



419 



and the rudimentary heart commences to beat. 

 No glycogen cells can be detected at this time, 

 but about the beginning of the sixth day tliey 

 appear as minute dark brown spots after the 

 application of the dilute iodine solution. The 

 glycogen cells increase in size and become 

 more granular during the further development 

 of the fish. At the time of the appearance of 

 the cells the embryos are covered with a single 

 layer of very thin flat epithelium. The inter- 

 cellular substance of the epithelial cells can be 

 easily and strikingly stained, after the appli- 

 cation of a weak iodine solution and wash- 

 ing in water, by immersing the animal in a 

 dilute aqueous solution of methylene blue for 

 a short time. The blue staining fluid forms a 

 dark precipitate with the iodine in the cement 

 substance, and forms zigzag lines which de- 

 limit the cells in the clearest manner. The 

 dark lines may be seen to cross the glycogen 

 cells in many places, indicating that these 

 cells are beneath the epithelial covering. 



The glycogen cells are usually more or less 

 elliptical and their dimensions vary with their 

 stage of development. When they first appear 

 their diameters may vary within the limits of 

 Sfi and 10/x. At this time the protoplasm of 

 the cells forms a ring surrounding a large 

 central vacuole containing the glycogen gran- 

 ules. One part of the ring is usually thick- 

 ened, and contains an elongated elliptical or 

 crescentic nucleus. As the cells enlarge with 

 the advanced development of the fish their 

 vacuoles encroach on the protoplasm until the 

 cells are converted into microscopic sacs of 

 glycogen, in the walls of which a long ellip- 

 tical or reniform nucleus can usually be 

 found. At this stage the diameters of the 

 cells may be IS^u, to 25/a, and the granules, 

 stained a mahogany color with iodine, are 

 chiefly found just below the cell membrane. 

 A number of these granules may coalesce and 

 form a rod-shaped body in the interior of the 

 cell. Sometimes three of the rods unite in 

 the shape of a Y. The stained granules of 

 glycogen dissolve with considerable rapidity 

 in the water containing the preparation, and 

 many of them disappear after a few minutes, 

 leaving the thin cell membrane containing the 



nucleus. A very weak solution of iodine 

 formed by adding a drop or two of the tinc- 

 ture to 5 c.c. of water gives the cells their 

 characteristic color in a few seconds if the 

 animal has been removed from the egg en- 

 velopes. If the embryos retain their gela- 

 tinous envelope they are stained in a few 

 minutes, and it is easy to follow the gradual 

 staining of the cells before the animals are 

 killed by the iodine. , 



At the time of the first appearance of the 

 glycogen cells there are no blood globules in 

 circulation, but these are first seen a day or 

 two later. At a little later period the liver 

 is formed, and may be stained a brick red by 

 the iodine solution. The liver cells do not 

 contain glycogen granules but are diffusely 

 stained a lighter and more reddish color than 

 the subepithelial glycogen cells. 



After a certain degree of development of 

 the fish the number of tlie glycogen cells be- 

 comes gradually lessened by absorption. As I 

 have had the perch under observation for only 

 a limited time after hatching I have never 

 witnessed the complete disappearance of the 

 cells. In and after the third week of devel- 

 opment their number becomes much smaller. 

 At that time the glycogen cells of the tail may 

 be crowded into its edge, and those of the 

 pectoral fins arranged in columns radiating 

 in the direction of the striation. This change 

 in position is probably due to the growth of 

 other tissue elements which displace the gly- 

 cogen cells. In advanced development 1 have 

 noticed in the tail fin many smaller meso- 

 blastic cells which are not stained with iodine. 



I have found many glycogen cells, very 

 similar to those of the yellow perch, in re- 

 cently hatched pike-perch or wall-eyed pike, 

 and in the small-moutlied black bass, but some 

 differences in the appearance of the cells in 

 the different species, and in the solubility of 

 their . glycogen granules were noted. The 

 glycogen cells of the pike-perch are coarsely 

 granular, and their glycogen dissolves very 

 rapidly in the dilute iodine solution. The 

 nuclei of the cells are not so apparent as 

 those of the yellow perch. The glycogen cells 

 of the pike-perch may be seen under the 



