Abstract
Summary By centrifuging fertilized, mature Ilyanassa eggs in raffinose solution, three cytoplasmic zones were produced, and the eggs were pulled into light and heavy fragments of approximately equal size. Light halves, which contained the lipid cap, intermediate zone, pronuclei, and only a small portion of the original yolk of the egg, could undergo further development. Two classes of light halves, animal and vegetal, were produced, and their development was compared. Animal halves were produced by centrifuging directly in raffinose solution, without prior treatment. Since Ilyanassa eggs orient under gravity with the vegetal pole downward, the light halves of unrestrained eggs represented the animal hemisphere. As a rule, these cleaved equally without formation of a polar lobe, and differentiated into partial larvae which lacked lobe-dependent structures (eyes, foot, shell, intestine, heart, etc.). A few exceptional cases are considered to have contained a portion of the vegetal hemisphere. Light halves containing all or part of the vegetal hemisphere were produced by a two-step centrifugation treatment. Eggs were first held upside down in a gelatin-seawater gel, and centrifuged until stratified internally. They were then freed from the gel and recentrifuged in raffinose solution to separate them into halves. In eggs with complete reversal of the usual stratification pattern, the nucleated light half would contain all the vegetal hemisphere; light havles of incompletely reversed eggs would contain a part of the vegetal hemisphere. Nucleated vegetal fragments, free of most of the yolk, formed a polar lobe of approximately normal proportions and underwent unequal first cleavage. Approximately half of the vegetal fragments differentiated lobe-dependent structures. It is concluded that the morphogenic influence of the vegetal polar area (polar lobe region) probably does not depend upon the presence of the yolk particles normally found there, and is able to withstand centrifugal force sufficient to displace lipid droplets, yolk particles, and nuclei.
Published Version
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