Internal Phenomena of Segmentation.
Numerous descriptions have been given during the last few years of the internal phenomena of segmentation. The most recent contribution on this head is that of Fol (No. [87]). He appears to have been more successful than other observers in obtaining a complete history of the changes which take place, and it will therefore be convenient to take as type the ovum of Toxopneustes (Echinus) lividus, on which he made his most complete series of observations. The changes which take place may be divided into a series of stages. The ovum immediately after the fusion of the male and female pronucleus contains a central segmentation nucleus.
In the first stage a clear protoplasmic layer derived from the plasma of the cell is formed round the nucleus, from which there start outwards a number of radial striæ, which are rendered conspicuous by the radial arrangement of the yolk granules between them. The nucleus during this process remains perfectly passive.
In the second stage the nucleus becomes less distinct and somewhat elongated, and around it the protoplasmic layer of the earlier stage is arranged in the form of a disc-shaped ring, compared by Fol to Saturn’s ring. The protoplasmic rays still take their origin from the perinuclear protoplasm. This stage has a considerable duration (20 minutes).
In the third stage the protoplasm around the nucleus becomes transported to the two nuclear poles, at each of which it forms a clear mass surrounded by a star-shaped figure formed by radial striæ. The nucleus is hardly visible in the fresh condition, but when brought into view by reagents is found to contain many highly refractive particles, and to be still enveloped in a membrane.
In the fourth stage the nucleus when treated by reagents has assumed the well-known spindle form. The striæ of which it is composed are continuous from one end of the spindle to the other and are thickened at the centre. The central thickenings constitute the so-called nuclear plate. The clear protoplasmic masses and stars are present as before at the apices of the nucleus, and the rays of the latter converge as if they would meet at the centre of the clear masses, but stop short at their periphery. There is no trace of a membrane round either the nuclear spindle or the clear masses; and in the centre of the latter is a collection of granules. The striæ of the polar stars are very fine but distinct.
Between the stage with a completely formed spindle and the previous one the intermediate steps have not been made out for Toxopneustes; but for Heteropods Fol has been able to demonstrate that the striæ of the spindle and their central thickenings are formed, as in the case of the spindle derived from the germinal vesicle, from the metamorphosis of the nuclear reticulum. They commence to be formed at the two poles, and are then (in Heteropods) in immediate contiguity with the striæ of the stars. The striæ gradually grow towards the centre of the nucleus and there meet.
In the fifth stage the central thickenings of the spindle separate into two sets, which travel symmetrically outwards towards the clear masses, growing in size during the process. They remain however united for a short time by delicate filaments—named by Fol connective filaments—which very soon disappear. The clear masses also increase in size. During this stage the protoplasm of the ovum exhibits active amœboid movements preparatory to division.
In the sixth stage, which commences when the central thickenings of the spindle have reached the clear polar masses, the division of the ovum into two parts is effected by an equatorial constriction at right angles to the long axis of the nucleus. The inner vitelline membrane follows the furrow for a certain distance, but does not divide with the ovum. All connection between the two parts of the spindle becomes lost during this stage, and the thickenings of the fibres of the spindle give rise to a number of spherical vesicular bodies, which pass into the clear masses and become intermingled with the granules which are placed there. The radii of the stars now extend round the whole circumference of each of the clear masses.
In the seventh stage the two clear masses become elongated and travel towards the outer sides of their segments; while the radii connected with them become somewhat bent, as if a certain amount of traction had been exercised on them in the movement of the clear masses. Shortly afterwards the spherical vesicles, each of which appears like a small nucleus and contains a central nucleolus, begin to unite amongst themselves, and to coalesce with the neighbouring granules. Those in each segment finally unite to form a nucleus which absorbs the substance of the clear mass. The new nucleus is therefore partly derived from the division of the old one and partly from the plasma of the cell. The two segments formed by division are at first spherical, but soon become flattened against each other. In each subsequent division of these cells the whole of the above changes are repeated.