Sea urchin. Fertilization and cleavage. Normal speed. Time-lapse.

The sea urchin egg is characterized by the homogeneous distribution of deutoplasm and thus it is particularly suited for observation of total and equal cleavage. To the right an immature still diploid ovum with large nuclear vesicle and nucleolus.

To the left a mature haploid egg with a poorly defined nucleus. The spermatozoa have three distinct morphological regions. The conically pointed head with the acrosome and nucleus, the middle piece with the centrosome and numerous mitochondria and the tail piece acting as a flagellum.

The sperm swim actively towards the egg cells. The first spermatozoan to reach the surface of the egg penetrates it immediately. At this moment the ovum is blocked for further sperms. Starting from the point of sperm attachment, the vitelline membrane rises from the egg to form the fertilization membrane.

Cortical granules flow into the fluid filled perivitelline space which surrounds the egg after about a minute. Here a sperm is just entering the egg.

When the fertilization membrane rises from the surface, the egg cortex contracts at the point of entry showing a temporary indentation. Under the interference contrast microscope, karyogamy is rendered visible. First the sperm centrosome in the form of an astrosphere

approaches the female pronucleus here from the left. Soon the male pronucleus becomes visible. Both pronuclei migrate towards each other, the male pronucleus covering the greater distance. About 40 minutes following sperm entry, the two pronuclei fuse to form the cincarion.

The next stage of development, the prophase, starts with the division of the centrosomes. As the two daughter centrioles separate, a cytoplasmic configuration described as the streak appears, which later gives way to the spindle fibers, the so-called amphiaster, during metaphase. The first cleavage furrow is formed perpendicular to the spindle axis.

Time-lapse cinematography demonstrates the cleavage of four cells up to blastulation. The morphogenesis of the echinoderm egg is characterized by its total and mainly equal cleavage.

The divisions take place at more or less regular intervals, so that after about five hours, eight divisions have been accomplished, resulting in a hollow sphere consisting of 256 blastomeres.

Cilia are developed round the periphery, and the blastula rotates within the fertilization membrane. About seven hours after insemination, the fertilization membrane splits, releasing the blastula to a planktonic mode of life.

The first cleavage is under stronger magnification. The first two cleavage planes are meridional, the third equatorial. Four animal and four vegetal blastomeres are differentiated. The fourth, partly unequal cleavage gives rise to four micromeres at the vegetal pole.

The individual tiers of cells now surround a spherical cavity called the blastocoel. Starting from the fertilized egg, morphogenesis is now demonstrated schematically

up to the 64 blastomere stage. The sea urchin egg exhibits polar symmetry, the animal and the vegetal areas being orientated in graded polarity. The coloring, yellow for animal tendency, green and red for vegetal tendency. This will serve to indicate the subsequent fate of various prospective areas.

The regular cleavage pattern of the sea urchin zygote is due to a characteristic change in orientation of the mitotic spindle plane to the polar axis of the egg. At the first cleavage, the spindle is horizontally aligned on the equatorial plane of the zygote. So the first cleavage is meridional

and leads to the segmentation of equally sized blastomeres. Before the onset of the second cleavage, the two spindles are again aligned on the equatorial plane, but now they are turned at 90 degrees to the plane of projection. The second cleavage, like the first, is meridional and equal.

It results in a tetrad of four equivalent blastomeres. Before the onset of the third cleavage, the spindle axes are vertically orientated, that is perpendicular to the equatorial plane. The third cleavage is affected equatorially and produces the eight blastomere stage

in which, for the first time, animal and vegetal cell material is separated by cell membranes. The heterogeneous differentiation of the two polar tiers of cells is made evident by their varying cleavage planes. Before the fourth cleavage, the spindles of the animal mesomeric tier

are horizontally orientated. The spindles in the vegetal macromeric tetrad, on the other hand, are eccentric and pointing obliquely to the vegetal pole. The fourth cleavage results in the 16-cell stage. By equal and meridional cleavages,

the animal pole has now differentiated a tier of eight equally sized mesomeres. In the vegetal hemisphere, the specific spindle alignment has conditioned an unequal, approximately horizontal, cleavage plane. Thus, the vegetal hemisphere consists of an equatorial tier of four large macromeres,

from which four considerably smaller micromeres have segmented towards the vegetal pole. The following cleavages take place alternately on the equatorial and the meridional planes. Thus, at the fifth cleavage, the eight animal mesomeres are divided by equatorial cleavage

into two tiers of almost equal size. The four macromeres, conversely, divide meridinally, resulting in a vegetal tier of eight equally sized cells. On account of a somewhat retarded micromere cleavage, the early blastula passes through a 28-cell stage before reaching the actual 32-cell stage with eight macromeres.

At this stage, the early blastula is differentiated into four regions, the two animal cell tiers 1 and 2, the vegetal macromere tier, and a total of eight micromeres at the vegetal pole. The sixth cleavage is again in two phases,

resulting in a 56- and a 64-cell stage. Now the position of the two animal tiers of cells becomes irregular. They ultimately form a rounded cap. Also in the vegetal hemisphere, by horizontal cleavage of the eight macromere,

both vegetal tiers of cells 1 and 2 have been differentiated. As cleavage proceeds, the original size difference between macro and micromeres is gradually equalized by decelerated division of the micromeres. These shots show the developmental stages in real life.

During the fourth cleavage, that is, from the eight to the 16-cell stage, the four micromeres are in course of differentiation, here to the right.

The subsequent fifth cleavage follows that of the micromeres after an interval. The rhythmical movements of the blastomeres are due to the almost synchronous cleavages. As it proceeds, the blastocele becomes more and more prominent. The micromeres are no longer visible.

Following the eighth cleavage, that is, upon reaching the 256-cell stage, the cilia are formed, and the blastula begins to rotate within the fertilization membrane.

The release of the blastula is filmed in three phases at normal speed. The blastula produces a hatching enzyme which partially dissolves the fertilization membrane so that the embryo can escape.

The blastula is flattened at the vegetal pole and swims with the apical end pointing forwards.