. Bonner zoologische Monographien. Zoology. 9. : Scanning electron micrographs of gametes of Branchiostoma Icmceolatum. A - External aspect of late oocytes, dissected from ovary. B - Spermatozoon. Note the three regions of the cell body corresponding to the acrosome, the nucleus, and the mitochondria (from apex to base). In Branchiostoma lanceolatiim the small eggs (~ 120 |um in diameter) appear to be without a micropyle (; Holland & Holland 1989). The mature male game- te is a "primitive" aquasperm (sensu Franzen 1956), consisting of an apical acro- some, a prominent,
. Bonner zoologische Monographien. Zoology. 9. : Scanning electron micrographs of gametes of Branchiostoma Icmceolatum. A - External aspect of late oocytes, dissected from ovary. B - Spermatozoon. Note the three regions of the cell body corresponding to the acrosome, the nucleus, and the mitochondria (from apex to base). In Branchiostoma lanceolatiim the small eggs (~ 120 |um in diameter) appear to be without a micropyle (; Holland & Holland 1989). The mature male game- te is a "primitive" aquasperm (sensu Franzen 1956), consisting of an apical acro- some, a prominent, roundish nucleus, few mitochondria, and a long motile fla- gellum, typical for marine invertebrates with external fertilization (; Baccetti et al. 1972). The zygote fornis a fertilization membrane (details of these early events are given by Holland & Holland 1992) and sinks to the bottom, where early development takes place. Secured in this fertilization membrane develop- ment proceeds through radial holoblastic cleavage to the blastula stage and furt- her on through gastrulation to the early neurula stages. The late gastrula stages begin to rotate within the fertilization membrane; when a neurula stage with two to three myotomes is reached, the embryos break free. Depending on the tempe- rature this takes place 15-18 hours post-fertilization (see Drach 1948). Locomotion in these early stages is exclusively powered by their long ectodermal cilia. During forward swimming these stages rotate alternatively clockwise or counter-clockwise. Bone (1958) noted that rotation in a counter-clockwise direc- tion (viewed from behind) is more common. Contrary to other reports (e. Bone 1958, Roschmann 1975) it seems most likely that development thereafter pro- ceeds entirely in the plankton (Courtney 1975, Stokes & Holland 1995b, perso- nal observation). The neurula flattens laterally and elongates anteroposteriorly, in addition, it acquires the ability to move by lateral und
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