Home >> Papers-from-the-department-of-marine-bio-volume-11 >> Properties Of Photogenin And to With Special Reference To_2 >> Study of an Embryo_P1

Study of an Embryo 20 Millimeters in Length

surface, dorsal, electric, vacuoles and striations

Page: 1 2

STUDY OF AN EMBRYO 20 MILLIMETERS IN LENGTH.

The head of the 20 mm. embryo was sectioned serially and trans versely and reconstructed in 1 mm. wax at a magnification of 500. This embryo was so well preserved that a very careful study has been made of it (plates ii and m).

The electric area has become definite electric tissue and is now seen for the first time on the obliquus superior muscle, making it evident that the electric tissue is generated from muscles which are innervated by the fourth and sixth nerves as well as from some which are inner vated by the third nerve. The three rectus muscles which contribute to the electric tissue have a common origin in the subcranial canal, the rectus inferior muscle having a separate origin. The obliquus superior arises at some distance from the three rectus muscles, but is the more dorsal of the two obliquus muscles. The fourth and sixth nerves take no part in the innervation of the adult electric organs. The organs are exposed dorsally, on the other hand, and only those muscles which are nearest the dorsal surface of the head have contributed to their formation, the two inferior muscles on the ventral surface not being concerned. Torpedo is the only other fish known to have the electric organs on the exposed or the dorsal surface, even though it occupies all the space between the dorsal and ventral surfaces. This tendency to develop the electric organs on the exposed or dorsal surface may be compared with the opposite tendency of fishes and other animals to develop their luminous organs on the ventral or protected surface, as shown in Porichthys. This correlation of the protective organ with the exposed surface and the attractive organ with the protected surface may be the result of natural selection, even though Darwin (37) con sidered electric organs to be one of the greatest difficulties in the way of his theory.

The electric cells have assumed the form of electroplaxes (plate v, fig. 4). They have enlarged to about six times the diameter of the ordi nary muscle-cells; and the large and numerous nuclei have rounded up close to the membrane, although a few remain scattered in the cyto plasm. The cytoplasm itself has become peculiarly vacuolated and

very definite striations have appeared. In longitudinal section each electroplax shows the ventral end indented into long, finger-like pro cesses (plate vi, fig. 1). The striations across these processes are continuous, as though the cell had been cut through at this point after the striations had been formed instead of the processes having grown out. The striations are perfectly definite and pass straight across the cell with no indication of the myofibrillation. Long, narrow vacuoles appear at intervals in the cytoplasm of the indented or evaginated end, while the dorsal end of the cytoplasm is filled with vacuoles of a smaller, rounder nature. This indentation is the beginning of the formation of the ventral papillae of the adult electroplax.

Vacuoles are formed in cells when changes in surface tension occur such that the pressure inside the cell-wall is greater than that of the surrounding medium. The electroblasts are growing at an abnormal rate in the longitudinal direction and not at all in the other direction, so that the pressure inside the cell must be tremendous. If this ten sion reaches a certain point, the vacuoles may increase in size until they burst through the cell-wall. That this is happening in these electro blasts is shown by the long, narrow shape of the vacuoles at the ven tral end, where many have already broken through to form the papillae. This explains why the striations at this end appear to be broken across instead of being separate striations formed for each papilla. It also explains why in the adult electroplax many of the papillae have con necting bridges of the same material as themselves (Dahlgren as), for in such cases the vacuoles have merely persisted instead of breaking through. The tension at the ventral end far exceeds that of the dorsal end, for the small, round vacuoles finally disappear without breaking through the wall and the dorsal surface becomes straight and smooth, the dorsal cytoplasm homogeneous and unvacuolated.

Page: 1 2