"THE STICKLEBACK'S EGG" "PART 1" "When the ripe egg is deposited in water, it is still polygonal." "The yolk is liquid and contains freely moving oil droplets confined to the upper part of the egg." "The spermatozoa penetrate through a funnel-shaped micropyle dug into the chorion (Zona radrata)." "Front view." "Note the canalicular passages in the chorion (dark striations)." "Side view." "After the ripe fertilized or unfertilized egg has been in contact with water for several minutes its cytoplasm undergoes a series of contractions." "These contractions occur in different directions." "They take a few minutes to travel from one end of the egg to the other and sometimes occur quickly enough to overlap." "A contraction wave upsets the shape of the egg, but once it has passed, little change is seen in the topography of the various elements." "ln accelerated motion, you see how violently the cytoplasm is churned." "You can still see the micropyle close to the germinal disc." "As the contractions continue, more often than not toward the germinal disc, the latter increases in volume and becomes more salient." "Soon, contraction is restricted to the germinal disc." "You can see the contraction and movement of the cytoplasm in the germinal disc, here greatly magnified." "Under the base of the germinal disc, the yolk is pushed back due to a neighboring cytoplasmic contraction not visible on-screen." "Contractions stop when the germinal disc is at maximum protrusion and looks like a nearly hemispherical cap." "The unfertilized egg degenerates." "If the egg is fertilized, cleavage begins roughly 30 minutes after contractions have stopped (at 20^C)." "Blastomere stages 2, 4 and 8." "The first furrow appears and divides the germinal disc into two blastomeres." "The latter swell and spread out." "The second division furrow then appears on each of the two blastomeres, etc." "(Note how the cytoplasmic churning in each blastomere" "has its own particular movement.)" "During segmentation, contraction is sometimes observe, d in the base of the blastomeres showing that the cytoplasm remains contractile during cleavage." "About 24 hours after fertilization when the blastoderm cap covers 215 of the egg, generalZi ed contractions resume but are limited to the cap." "The blastoderm cap covers 213 and eventually 415 of the egg." "The cytoplasmic contractions continue when the embryo is formed." "They involve the wall of the yolk sac, as opposed to the differentiated embryonic organs, which do not contract." "Note the cytoplasmic contraction of the extra-embryonic coelome wall on either side of the head of the embryo." "The isolated contraction of the outer coelomic layer is clearly visible." "Here, the two coelomic walls contract in unison to form a transparent cytoplasmic cushion on the egg's surface." "When muscular movements and circulation first appear, the last of the cytoplasmic contractions occur." "(The actual movement and contraction of the heart are here accelerated 20 times.)" "PART 2 BLOOD Circulation" "The heart (its atrium spread out like a funnel outside the embryo - observe the embryo tail beats before circulation begins." "Blood cells emigrating from the embryo inundate the yolk surface, forming very dense layers in certain areas." "(Varying the focus reveals first the red cells and then the heart beating without red cells.)" "The heart, which can push back a blood wave, aspirates proximal red cells." "It also pushes the mass of red cells developed in the embryo on the yolk sac via the caudal vein and the ducts of Cuvier." "This mass of red cells advances slowly and unhampered, creating a dense layer that approaches the heart." "Little by little, the crowding recedes, and the first solely lacunar current coming from the left side of the head begins to form." "The first red cells from the ducts of Cuvier reach the venous sinus." "This is the end of the trip for the first venous vascular circulation via the caudal vein and the trunk vein." "Circulation via the caudal vein the S-shaped vitelline vein and the venous sinus can start before the previous circulation." "ln any case, both circulations meet near the venous sinus." "(The embryo is seen from the ventral side, through the yolk sac)." "Soon, in addition to venous circulation in the yolk, arterial blood comes into play via development of mesenteric arteries." "(Here, blood from the posterior mesenteric artery and the caudal vein mix at the entrance of the vitelline vein.)" "Arteriovenous circulation (or second vitelline circulation) having been established, more blood is brought to the heart." "The spurts of blood produced by the contractions of the embryo distend the very thin wall of the vitelline vein in bursts at its upper bend." "When the egg hatches, a very tight network of capillaries brings blood from the mesenteric arteries to the large, previously established venous ducts." "The ducts of Cuvier can be seen to join the vitelline vein." "Hatching through the yolk sac." "The vitelline vascular network outside the chorion is clearly visible." "The chorion, shrunk in volume after the egg is hatched, is seen next to the embryo." "Caudal circulation." "At the tip of the tail, the caudal aorta turns around to become the caudal vein." "The intermyotomic segmental loops are formed." "Note how the supra- and sub-intestinal veins arise from the caudal vein." "Further on, the aorta and the caudal vein cross and form a loop." "In this particular area, the jerky circulation in the aorta becomes almost smooth in the vein."