174 Papers from the Marine Biological Lahoratoryat Toriiigas. 



size. The one selected for illustration is from a lower median spindle and 

 shows an actual length of 0.9 mm. It appears with smooth, well-defined 

 boundaries and good separation of core and outer layer. The myofibrils 

 are shown in the core as several closely associated bundles, only to be 

 distinguished from one another by a slight curving and twisting in their 

 course. This would not be visible in a transverse section at most levels in 

 the electroplax, and it gives strength to the view, expressed before, that the 

 fibril bundles of a number of young muscle cells combine to form the single 

 fibrous core of each electroplax. At the levels examined it would appear 

 that a group of from 4 to 7 cells from each myotome is concerned in the 

 formation of any electroplax and that these same groups of from 2 or 3 

 myotomes are likewise united end to end, thus making it possible that 

 from 12 to 21 muscle cells are united to form each electroplax. 



The beginning of the loss of transverse striation is quite visible in figure 

 17, plate 6. This striation merely fades or loses its staining power, beginning 

 at several points, but usually at the middle of the electroplax. The ends 

 retain the staining power of the M stripes longest. 



The multiplication of nuclei must be spoken of at this point. It is 

 well known that the nuclei of future muscle tissue divide by mitosis with a 

 consequent and subsequent division of the cell-body as long as the cells are 

 in the young myoblast stage. As soon as muscle differentiation begins to 

 take place, the mitotic division of nuclei is changed to an amitotic type of 

 division and the cell-body ceases to divide and begins to lay down myo- 

 fibrils. Some few observations against this view are recorded, but it seems 

 to hold in Gymnar chits. 



In figure 15, plate 5, one can readily see many cases of amitotic nucleus 

 division. The few cases of m.itosis visible in the figure are of connective- 

 tissue cells. Such cells, it is known, always divide by mitosis during their 

 whole existence. As the electroplax grows, more nuclei are needed, and they 

 are supplied by the amitotic divisions above mentioned. Just how long this 

 process keeps up is not known, but it is probably mostly done before the 

 first 15 or 20 days of development are passed. It is evidently going on 

 fast in the 12-day embryo, and it is possibly finished in the 42-day stage. 

 The nuclei are typical muscle nuclei in appearance and are not to be dis- 

 tinguished from the nuclei of real and active muscle in the same preparations, 

 except, perhaps, they are slightly larger and have a somewhat heavier chro- 

 matic content (larger nucleolus and chromatic granules). This does not 

 hold, of course, for the degenerating nuclei, whose differences have already 

 been described. Connective-tissue nuclei can at once be distinguished by 

 their delicate outline and small chromatin content, which is distributed as 

 very small granules. 



STUDIES OF A LARVA FORTY-TWO DAYS OLD. 



In order to continue tracing this history of the development and growth 

 of the electric tissue, we will now be obliged to pass to an embryo of some 



