*Much of the material on which this section is based was obtained from Dr. James M. Moulton, to whom we are most grateful.

LIVING MATERIAL

Two species (and probably several sub-species) of the common silversides are available in the Woods Hole, Mass., area. Menidia is characterized by the presence of a longitudinal silvery stripe, which runs along the flanks of the adult.

It is difficult to distinguish between Menidia menidia and M. beryllina solely on the basis of external morphology of the adults, although M. beryllina is somewhat smaller than M. menidia and has a colorless peritoneum, as opposed to the black peritoneum of M. menidia. The eggs of M. beryllina are somewhat smaller than those of M. menidia, and each has 8 to 14 attaching threads per egg, in contrast to the larger number (about 40) characteristic of the egg of M. menidia. All the attaching threads of the M. menidia egg are of approximately equal size, but Moulton (personal communication) reports that one thread of the egg of M. beryllina is notably larger (by a factor of two or three) than the remaining ones. Further details may be found in the book by Breder (1948).

During the breeding season, the females of both species are considerably plumper than the males, but other criteria for distinguishing between the sexes are somewhat unsatisfactory. Often, the females in a school of Menidia out-number the males (Kendall, 1901).

The adults were formerly very abundant in the Eel Pond, and some are still available there, as well as at other collecting sites (frequently in the same locales as Fundulus). Kendall (1901) reported that Menidia was common at that time about the wharves in Great Harbor, at Woods Hole.

From mid-June to mid-July; the last two weeks in June are probably most favorable (Moulton, personal communication). Bumpus (1898) reported that eggs are also obtainable early in June.

A. Care of Adults: The fish live well in aquaria supplied with running sea water, but it is important that they be transferred to such aquaria as soon as possible after collection.

B. Procuring Gametes: Eggs and sperm are obtained by stripping the fish (see the section on Fundulus, p. 224 of this manual). Some immature eggs (which lack the characteristic attachment threads) are usually obtained from the females; such eggs are pale in color and smaller than ripe eggs, and do not tend to cling together as do mature eggs.

C. Preparation of Cultures: Eggs may be inseminated by the same general methods described for Fundulus. The cultures are best kept on the sea water table; a temperature of 18 to 19û C. is apparently most favorable (Moulton, personal communication).

D. Methods of Observation: The attachment threads may be cut off close to the egg surface, using a sharp scalpel or razor blade. (See, also, the methods used for observation of Fundulus eggs.)

A. The Unfertilized Ovum: The egg of M. beryllina is approximately 0.75 mm. in diameter (Breder, 1948), while that of M. menidia is somewhat larger and measures about 1.2 mm. in diameter (Nichols and Breder, 1927). The eggs of both species are clear and somewhat yellowish in color, and two to three oil droplets (which later coalesce into one) are present. The attachment threads arise from a very circumscribed area of the chorion, 180 degrees from the future site of origin of the blastodisc (Moulton, personal communication).

B. Fertilization and Cleavage: The sperm enters the egg through a micropyle. Polar bodies have not been observed in developing Menidia eggs (Moulton, personal communication), and the stage of the egg nucleus at the time of fertilization is not known. Formation of the germinal disc and cleavage are, in general, similar to the same processes in the Fundulus egg. During the course of development, the egg of Menidia becomes free within the chorion, so that the position of the attachment threads is no longer a criterion of the polar axis.

C. Time Table of Development: The development of Menidia eggs is slow; Moulton (personal communication) observed the following schedule, at a temperature of 18-19û C. The times are recorded from insemination.

Stage Germinal disc First cleavage Eight to 32 cells Beginning of expanding blastula Early embryonic shield; germring halfway around yolk Beginning of gastrulation Yolk plug; optic vesicles to closed blastopore Eyes formed; heart beating Hatching

Time By 40 minutes 60 minutes ** 3 hours, 50 minutes 16 hours, 20 minutes 27 hours, 35 minutes 36 hours 39 hours, 35 minutes 6 days 15 days

D. Later Stages of Development: The later development is like that of Fundulus, except that by four days, the eggs are clear and transparent, so that observation of the embryo is easier. The large oil droplets coalesce approximately 16 hours after insemination, to form a single drop. At six days, the eyes are well formed, the heart is beating and Kupffer's vesicle is clearly visible. The circulatory system in a 48-hour embryo is diagrammed by Clark and Moulton (1949). Shortly before hatching, the chorion becomes very soft and flabby.

** From the paper by Bumpus (1898); the temperature is not specified.

Figures of many stages in the development of M. menidia are available in the paper by Kuntz and Radcliffe (1917).

SPECIAL COMMENTS:

The localization of the egg attachment threads to a circumscribed area, together with the small number of oil droplets, facilitate study of this form, especially in early stages (Clark and Moulton, 1949). Thus, the eggs of Menidia have some advantages over those of Fundulus, for both study and experimentation. In addition, the spawning season of Menidia is usually somewhat more prolonged than that of Fundulus.

BREDER, C. M., JR., 1948. Field Book of Marine Fishes of the Atlantic Coast from Labrador to Texas. G. P. Putnam's Sons, New York. (Rev. ea.)

BUMPUS H. C., 1898. The breeding of animals at Woods Holl during the months of June, July and August. Science, 8: 850-858

CLARK, E., AND J. M. MOULTON, 1949. Embryological notes on Menidia. Copeia, 1949, no. 2, pp. 152-154.

KENDALL, W. C., 1901. Notes on the silversides of the genus Menidia of the east coast of the United States, with descriptions of two new subspecies. Rep. of the Commissioner, U. S. Comm. of Fish & Fisheries, 27: 241-267.

KUNTZ, A., AND L. RADCLIFFE, 1917. Notes on the embryology and larval development of twelve teleostean fishes. Bull. U. S. Burl Fish., 35: 87-134.

NICHOLS, J. T., AND C. M. BREDER, JR., 1927. The marine fishes of New York and southern New England. Zoologica, 9: 1-192.