98 THE EPIBLAST. 



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 g;reater part of the body. In 

 Torpedo they present nearly the same characters as in Pristiurus 

 and Scyllium, but are somewhat more columnar. (PI. X. 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 imdivided 

 dorsal fin — and the cells forming this, unlike the general epi- 

 blast cells, are markedly columnar ; they nevertheless, here as 

 elsewhere, form but a single layer. (PL x. 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 com- 

 posing them are, however, not columnar, and the folds them- 

 selves are merely artificial products due to shrinking. 



