178 Papers from the Marine Biological Laboratory at Tortugas. 



statements are the result of averages of about 15 measurements. The 

 papillae on the posterior surface have increased in length and some of them 

 have begun to rival the posterior portion of the organ in length. We have 

 no suitable figures of the adult electroplax, but from Fritsch's (3) descrip- 

 tions, and from the tendencies shown by these embryos, it would seem that, 

 as Ewart describes in Raja, the original posterior portion of the electroplax 

 shortens and the new papillae lengthen until they all form approximately 

 similar structures. This can only be completely studied when we have 

 secured suitable sections of the grown fish. 



The development of papillae is noticeably weak on the anterior surface. 

 The figure does not show as many as some of the electroplaxes in figure 23, 

 plate 9, but it is a fair illustration. Neither does it show well the usual 

 condition of the main anterior process of the cell at this time, which can be 

 better seen, in some electroplaxes of figure 23, plate 9, to be still in evidence 

 and of considerable length, but of very weak development. This anterior 

 process shows no trace at this age of the general vacuolization of its cyto- 

 plasm which we saw in an earlier stage, and the fibrils extend as a very thin 

 and uncertain core through its length. 



The fibrillar mass which forms the core of the electroplax has now 

 assumed what appears to be its permament condition. The fibrils are 

 very fine and, after having passed straight down through the posterior 

 process, are thrown into flat waves by the process of packing them into 

 the shortening and widening middle part which now constitutes the principal 

 bulk of the structure. This causes the larger part of the fibrils to lie nearly 

 at right angles to the longitudinal axis of the organ, and this appearance 

 was first taken by the writer, who examined the oldest embryo first, to be a 

 trace of the transverse striation of muscle from which the organ is formed. 

 We now know it to be the myofibrils, lying at right angles to their original 

 course. No conclusion was arrived at, in this stage, concerning the growth 

 of this mass, as to whether the number of fibrils was increased, or whether 

 the larger size was due to a thickening of the original fibers, or to the deposit 

 of interfibrillar substance. The fibrils seemed to be as fine if not finer than 

 in the earlier stages; certainly they are much finer than functional myofibrils. 

 Since muscle tissue increases the number of its myofibrils long past this age, 

 I see but little reason why this modified muscle should not also do so. 



A closer study of the cytoplasm and nuclei of the peripheral layer was 

 next undertaken in this oldest stage. Beginning on the posterior surface 

 and on the papillae, we find the layer thickest here and composed of at least 

 three distinguishable materials. One was a dense material which was 

 reticular in structure and stained with chromatic stains deeper than almost 

 any other pure cytoplasm that I know of. This material was one con- 

 stituent, while the other substance composing the general field at this point 

 was a far lighter staining material which was homogeneous and clear. This 

 latter seemed to bear the same relation to the denser material that the 

 "nuclear sap" does to a linin alveolum or reticulum in the nucleus. Like 



