Papers from the Marine Biological Laboratory at Tortugas. 211 



but also, if at all possible, to watch the processes of growth and differ- 

 entiation as they were taking place. All attempts, however, to observe 

 the breaking down of the nucleus and the division of the centrioles proved 

 to be unsuccessful. These processes are completed very rapidly and it is 

 doubtful whether the centrioles with their radiations could be distinguished 

 intra vitam. On the other hand, however, such processes as the vesiculation 

 of the nuclear fragments, the formation of the albuminous bodies, and the 

 growth of the axial fibers take place so gradually that they showed no appre- 

 ciable progress in cells which were under constant observation for two or 

 three hours. Nevertheless, many different stages in the development of the 

 apyrene spermatozoa could be distinguished and observed most satisfactorily. 



The large nucleated cell, corresponding to the oligopyrene spermatocyte I 

 of Paludina, was easily recognized. Earlier stages in the growth of that 

 cell could not be distinguished from the eupyrene spermatocytes. The 

 next stage (fig. i) which could be easily picked out was one in which the 

 axial fibers had grown well across the cell and many of the nuclear frag- 

 ments had become vesiculated. Figure 2 shows a stage of approximately 

 the same age, but viewed at an angle of 90 from the other. In these stages 

 the cell is not much larger than it was just previous to the breakdown of 

 the nucleus. Later on, the continued growth of the axial fibers results in 

 the elongation of the cell; this makes it appear larger, but no real increase 

 in its volume has taken place. This is shown in figures 3 and 4, which 

 represent stages following closely one upon the other and viewed in planes 

 at right angles to each other. In figure 3 the bundle of axial fibers is seen 

 just in the act of splitting apart, while in figure 4 the process is represented 

 as being probably a little more advanced. 



This splitting of the bundle of axial fibers is followed by the further 

 separation of the halves until they reach the cell-wall on both sides. Very 

 soon they begin to project edgewise beyond the cell, giving the first indi- 

 cation of the formation of the undulating membranes (fig. 5). Beyond this 

 point there is not a very great increase, if any, in the actual volume of the 

 cell; it merely becomes gradually longer and narrower until the adult con- 

 dition is reached (fig. 6). It will be noticed that as the cell approaches 

 maturity it becomes more and more filled with the albuminous bodies, 

 while the nuclear fragments degenerate and disappear. When first formed, 

 the albuminous bodies are spherical in size and are scattered irregularly 

 throughout the cell, first filling up its anterior portion. Later on, as a 

 result of their increase in number and the lateral compression of the cell 

 due to its elongation, they become polygonal and arrange themselves in an 

 orderly fashion. Until the undulating membranes have been formed the 

 cell is immotile; the flagella at its base never beat, but instead undergo 

 fusion to a certain extent. As will be shown later on, the myoneme-like 

 striations are not present until after the formation of the membranes. 



The study of the living cells of the testis is very valuable, because by 

 this method all possibilities of abnormality due to fixation and subsequent 



