CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 317 



11. RESULTS OF CENTRIFUGING DURING MATURATION AND FER- 

 TILIZATION STAGES 



In Crepidula, as in most other prosobranchs, the eggs are fer- 

 tiHzed within the o\'iduct of the female and are then surromided 

 by secretions from the nidamental gland; the outermost layer of 

 these secretions hardens into a capsule. By opening the oviduct 

 of females taken in the act of egg-laying or by getting capsules 

 immediately after they have been deposited, it is possible to 

 obtain eggs before the germinal vesicle breaks down and before 

 the spermatozoon enters the egg. 



Under normal conditions the polarity of the unfertilized egg is 

 marked by the eccentricity of the germinal vesicle toward the 

 animal pole. Generally the spermatozoon enters the egg near its 

 vegetal pole, though there may be exceptions to this rule. Under 

 normal conditions both first and second polar bodies are formed 

 invariably at the animal pole; indeed so general is this rule that 

 the animal pole is frequently defined as that pole of the egg at 

 which the polar bodies are formed, and yet, as we shall see later, 

 the polar bodies may be caused to form at any point on the sur- 

 face of the egg without in any way changing the polarity of 

 development. In Crepidula the polar bodies never change their 

 point of attachment to the egg; as long as they are present they 

 remain at the point where they were extruded and they there- 

 fore constitute a valuable landmark. 



1. Results of centrifuging before and during the first maturation 



division (figs. 1-18) 



From the facts just stated it is evident that before the forma- 

 tion of the first polar body, there is no sure way of distinguishing 

 the original poles of a centrifuged egg of Crepidula, though the 

 incomplete stratification of the cell constituents, and particu- 

 larly the position of the sperm nucleus and of the first matura- 

 tion spindle and the direction of movement of various cell con- 

 stituents after centrifuging, may indicate with a certain degree 

 of probability the location of the original poles. 



