Boveri (1901) made the important discovery of the existence of prelocalisation. He observed that in the unfertilised ovum of Paracentrotus (Strongylocentrotus) lividus there was an evenly-distributed pigmentation. When fertilisation occurred, and maturation was completed, the pigment became restricted to a transverse band in the lower hemisphere of the egg. Subsequent development showed that it is this pigmented zone alone which normally is able to give rise to the archenteron; eggs which had been deprived of the pigmented zone were unable to gastrulate. Therefore it was evident that a prelocalisation of archenteron-forming elements had taken place in the egg, and that the transverse band of pigment indicated a vegetal hemisphere.

MacBride (1907), in the course of his careful study of the development of Ophiothrix fragilis was more concerned with later stages than the egg, and he merely notes in passing that the eggs of that species measure about 1 mm. in diameter, and are opaque owing to the presence of a yellow yolk. He does not refer to his having noted any evidence of prelocalisation. His earlier work on Amphipholis (1892), as already stated, concerned only post-larval stages. In a paper on the development of Echinocardium cordatum (1918) MacBride similarly makes little reference to the egg, but he mentions that it is an ellipsoid, not a sphere. Here again he does not refer to any prelocalisation.

Grave (1900) had to omit a description of the egg and early cleavages from his paper on the development of Ophiura brevispina, owing to lack of material. But in 1916 he was able to complete his previous work in a second paper. In this he gives a careful description of the egg, as follows: “The mature egg of Ophiura has a diameter of approximately 0.3 mm. and is therefore about seventy-eight times the volume of the egg of Ophiocoma echinata, the latter being not far from the minimum size of eggs produced by ophiuroids in general. The great difference in the size of the eggs of these two species seems to be chiefly, if not exclusively, due to a difference in yolk content, and not to a difference in volume of ground substance. The yolk material of the egg of Ophiura. is distributed uniformly throughout the cytoplasm in the form of minute spherules and does not become stratified or localised either before or after fertilisation.” Grave further states that the yolk spherules give the egg its characteristic colour—varying from green to lemon-yellow in different broods. His subsequent account of the early development does not refer to any prelocalisation of archenteron-forming material, nor did he find any correlation between pigment distribution and the fate of various regions of the egg.

Narasimhamurti (1933), working on the development of Ophiocomina nigra, states that the egg is small, measuring 0.1 mm. in diameter, and is opaque owing to the presence of a brownish yolk. He found no evidence of prelocalisation.

Smith (1940) has described the oogenesis of Ophiothrix fragilis. The ovary of this species is large, and has a special gonoduct to the

genital cleft. Within the gonad there are to be seen oöcytes which fall into well-defined size-groups. He believes that the sex-cells originate in the rachis, whence they migrate into the gonads. The youngest oöcytes observed had a vesicular nucleus, and a central or slightly excentric nucleolus. The latter is at first dense, but exhibits vacuolation when the oöcyte attains a diameter of thirty micra, and continues to do so till the ovum is 90–100 μ in diameter.

Fell (1941) described the directly developing ophiuroid of New Zealand known at present as “Kirk's ophiuroid.” In this species the egg is very large, having a diameter of 500μ, and is so densely laden with yolk as to be quite opaque, no internal structure whatever being visible without sectioning. The egg is uniformly coloured with a buff or pinkish-brown pigment which is intimately associated with the yolk material. The course of later development shows very clearly that in this species prelocalisation of endoderm-forming and epiblastic material occurs early in development, not later than the second cleavage, and possibly in the egg itself, as in Paracentrotus lividus. Unlike the latter species, however, prelocalisation cannot be correlated with pigment distribution. At first the yolk is evenly distributed through epiblast and mes-hypoblast alike, but later it disappears from the epiblastic cells, and becomes confined to the central mes-hypoblast tissues.

Fig. 3.—The Ovary.
Coel., coelom; Ovy.WI., ovary wall; Oöct., oöcyte; oögon, oögonia; WI.Burs., wall of bursa.

In regard to the oöcyte and egg of Amphipholis squamata, the following facts have been observed: Within the ovary, which is here very small—normally about 100μ across—there are generally to be seen a number of small, loosely-arranged oogonia, each with a prominent nucleus, and usually a small nucleolus to one side of the nucleus. Occasionally a more mature oogonium is to be seen in which the nucleus is somewhat enlarged, and the nucleolus occupies a more central position in the nucleus. This sequence is in agreement with the observations of Harvey (1931) on Asterias rubens. With increasing maturity the nucleolus tends to stain more deeply, a condition also paralleled in the oogenesis of Asterias rubens.

In addition to the oogonia there are usually to be seen in an ovary two oöcytes of considerable size, one of them twice or thrice as large as the other. In these oöcytes the nucleus has become greatly enlarged, thus taking on the typical form and translucency of the “germinal vesicle.” Near the centre of the nucleus a large nucleolus is to be observed. In the cytoplasm around the nucleus yolk-granules are present, and these grow denser and more evenly distributed throughout the egg as growth proceeds. When the egg is mature it is liberated directly into the bursa, the wall of which is thin in the region approximate to the ovary. Only one egg at a time comes to maturity in any one ovary as a general rule, though occasionally two embryos have been observed in one bursa of apparently the same age, indicating perhaps the contemporaneous liberation of two ova.

Fig. 4.—Ovum which has just been liberated into the bursa.
Nue., nucleus; Wall.Burs., wall of the bursa; Nucleol., nucleolus; Rup.Fol., ruptured follicle; Lum., lumen of the bursa.

In Fig. 4 is shown a mature unfertilised egg which has just passed into the bursa and still has part of the follicle adhering to it. It has a diameter of approximately 100μ, due allowance being made for some shrinkage during fixation. In life the egg is deeply tinted with a reddish opaque pigment which is associated with the yolk in the cytoplasm. The nucleus is large and clear, about 50μ in diameter, and there is a large and deeply staining nucleolus. Beneath the fragmentary portions of ruptured follicle and lying in close contact with the cytoplasm of the egg is a thin but distinct protoplasmic membrane, but in the unfertilised specimen shown in the figure there is as yet no egg (or “fertilisation”) membrane. The yolk material is in the form of numerous fine droplets, much smaller than those of the egg of Kirk's ophiuroid. As in the latter species, the yolk-granules are intensely basiphilic, but, although this property prevents the use of nuclear stains for whole mounts, it is not strong enough to interfere with their use for sections Thus, it was not found necessary to use the special staining methods which had to be employed for Kirk's ophiuroid, where the basiphilic granules tend