3l6 DEVELOPMENT OF ELASMOBRANCH FISHES. 



The cells which compose it are flattened and polygonal in 

 outline, but more or less spindle-shaped in section. They present 

 a strong contrast to the remaining embryonic cells of the body 

 in possessing a considerable quantity of clear protoplasm, which 

 in most other cells is almost entirely absent. Their granular 

 nucleus is rounded or oval, and typically contains a single 

 nucleolus. Frequently, however, two nucleoli are present, and 

 when this is the case an area free from granules is to be seen 

 around each nucleolus, and a dark line, which could probably 

 be resolved into granules by the use of a sufficiently high 

 magnifying power, divides the nucleus into two halves. These 

 appearances probably indicate that nuclei, in which two nucleoli 

 are present, are about to divide. 



The epiblast cells vary in diameter from '022 to '026 Mm. 

 and their nuclei from '014 to '018 Mm. They present a fairly 

 uniform character over the greater part of the body. In Torpedo 

 they present nearly the same characters as in Pristiurus and 

 Scyllium, but are somewhat more columnar. (PI. n, fig. 7.) 



Along the summit of the back from the end of the tail to 

 the level of the anus, or slightly beyond this, epiblast cells form 

 a fold the rudiment of the embryonically undivided dorsal fin 

 and the cells forming this, unlike the general epiblast cells, are 

 markedly columnar ; they nevertheless, here as elsewhere, form 

 but a single layer. (PI. n, fig. 3 and 5 df.} Although at 

 this stage the dorsal fin is not continued as a fold anteriorly 

 to the level of the anus, yet a columnar thickening or ridge 

 of epiblast, extending along the median dorsal line nearly to 

 the level of the heart, forms a true morphological prolongation 

 of the fin. 



On the ventral side of the tail is present a rudiment of the 

 ventral unpaired fin, which stops short of the level of the anus, 

 but, though less prominent, is otherwise quite similar to the 

 dorsal fin and continuous with it round the end of the tail. At 

 this stage the mesoblast has no share in forming either fin. 



In many sections of the tail there may be seen on each side 

 two folds of skin, which are very regular, and strongly simulate 

 the rudimentary fins just described. The cells composing them 

 are, however, not columnar, and the folds themselves are merely 

 artificial products due to shrinking. 



