Volume 1

A treatise on comparative embryology / by Francis M. Balfour.

  • Balfour, Francis M. (Francis Maitland), 1851-1882.
Date:
1880-1881
Catalogue details

Licence: Public Domain Mark

Credit: A treatise on comparative embryology / by Francis M. Balfour. Source: Wellcome Collection.

    93/540 (page 73)
    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 stria?. 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 striae 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 striae 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 strite 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 striae of the stars. The striae 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 dehcate 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 amoeboid 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 viteUme 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 thickenmgs 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 m the