Libinia emarginata may be distinguished from the less commonly found L. dubia by the presence on its back of nine median spines, as contrasted with the six median spines characteristic of L. dubia. The animals occur on mud flats, and are abundant. The eggs are carried by the females on the legs; those egg-masses which are bright vermilion in color are best for study, since they contain the early stages. Later stages of development are chocolate-brown in color. Zoea and megalops larvae (of this and other crabs) are sometimes obtained by towing in the Hole at Woods Hole, Mass. Frequently, an electric light shining near the surface of the Eel Pond water (as, for example, the Nereis-collecting light commonly used) will attract vast numbers of larvae, which may then be dipped up.

Bumpus (1898a) reported that females with eggs were collected at Woods Hole during the month of May and (1898b) that oviposition was observed as late as August 7. Thompson (1899) states that the larvae had disappeared from his collections by September 4.

A. Care of Adults: The animals should be kept in large aquaria supplied with running sea water.

B. Methods of Observation: Forceps may be used to pick the eggs and larvae from the legs of the female. Cleavage and subsequent stages of development can be studied in the living egg only with considerable difficulty, and it is therefore advisable to fix the embryos. The following methods are useful:

1. Place the eggs in strong aceto-carmine (in a very little sea water). After about ten minutes, transfer them to 50% glycerine, which causes the stain to fade.

2. Early embryos are readily studied after the addition of 1% chromic acid to a drop of sea water in which the embryos are contained. After about five minutes, wash the embryos in water and mount them on glass slides under mica coverslips. 3. Drop the older stages into strong mercuric chloride solution; the embryonic area will whiten in a few minutes.

Use reflected light and a low power of the microscope for observation of all stages which have been fixed.

Study of the zoea stage in the living condition requires that the larvae be anaesthetized (with magnesium sulfate or other agents). With the use of the higher powers of the microscope, such details as the muscles, compound eyes, contractile heart and intestine can be observed.

A. Early Stages of Development: The early development of the centrolecithal egg of Libinia is essentially the same as that of the crayfish, Astacus. Since the paper by Reichenbach (1886) is in a journal which is not readily accessible, this account is based upon the summary given by MacBride (1914), and on the description by Brooks and Herrick (1892) of the cleavage of Alpheus and Stenopus.

The zygote nucleus occupies a central position in the fertilized egg and there divides. Protoplasmic division is said not to begin until after the fourth nuclear division, by which time the nuclei have migrated to the periphery. The daughter nuclei are at first internal but gradually migrate outward until they reach the surface. At this time the egg is imperfectly divided (by radiating planes of cytoplasm between masses of yolk granules) into a series of pillars, each of which contains one of the daughter nuclei. These are referred to as "columnar blastomeres." The yolk pyramids persist for only a short time; then the dividing planes disappear, and a flattened "skin" of cells remains, surrounding a large mass of yolk. This "skin" of cells is termed a ''blastoderm.'' This stage corresponds to a blastula, the blastocoele being filled with an unsegmented mass of yolk.

Preceding the formation of the gastrula, there is an increase in the number of the blastoderm cells on one side of the egg; they are also thicker here and this becomes the ventral surface of the embryo. They press on one another laterally as they increase in number and become columnar in character, to form the ventral plate. This is on the future neural side of the embryo. Five circular areas develop in this ventral plate, in each of which the cells are arranged in concentric curves, and in lines radiating from a central point. These areas may be clearly distinguished in an embryo of this age after fixation with mercuric chloride, if it is examined by reflected light. The two anterior areas are the "cephalic lobes," or the rudiments of the paired eyes and cerebral ganglia. The thoracico-abdominal thickenings posterior to these constitute the next pair of rudiments. Just posterior to these, on the mid-line, is the central disc, or endodermic rudiment.

At the anterior margin of the endodermic rudiment, a groove develops. This is the beginning of the blastopore. The appearance of this groove (the endodermic groove) marks the beginning of the process of gastrulation. The endodermic groove later becomes a complete circle, as the periphery of the endodermal disc is invaginated, giving rise to the ''endodermal button." As the button is carried in, a circular blastopore forms, later changing into an elliptical blastopore. The front border of the endodermic rudiment is the point of origin of the mesoderm. The endodermal tube becomes pinched off as a blind sac; much later, the proctodeum and stomodeum grow through to it. The proctodeum appears in between, and just posterior to the thoracico-abdominal rudiments, where the blastopore formerly was located. Other embryonic areas develop a short time after the first five. Three of these are the rudiments of the anterior paired appendages: first antenna (antennule), second antenna, and mandibles, and are characteristic of all crustacean larvae. When these rudiments appear, the stage is called a nauplius.

B. Rate of Development: About one month passes between spawning and the zoea stage.

C. Later Stages of Development: The nauplius stage has the first three pairs of appendages; later stages have five or more pairs of appendages, and the stomodeum, ventral fold, dorsal shield, telson and ganglia may be found. It is necessary to supply the older larvae with Ulva and diatoms.

REFERENCES

BROOKS, W. K., AND F. H. HERRICK, 1892. The embryology and metamorphosis of the Macroura. Mem. Nat. Acad. Sci., 5: 325-574.

BUMPUS, H. C., 1898a. The breeding of animals at Woods Holl during the month of May, 1898. Science, 8: 58-61.

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

MacBride, E. W., 1914. Text-book of Embryology. Vol. I. Invertebrata. Macmillan and Co., Ltd., London.

REICHENBACH, H., 1886. Studien zur Entwicklungsgeschichte des Flusskrebses. Abh. Senckenberg. Naturforsch. Ges., 14: 1-137.

THOMPSON M. T., 1899. The breeding of animals at Woods Hole during the month of September, 1898. Science, 9: 581-583.