The Collaborative International Dictionary of English v.0.48:

Pro thyalosoma \Pro* thy`a*lo*so"ma\, n.; pl. Prothyalosomata.
   [NL., fr. Gr. prw^tos first + "y`alos glass + ?, ?, body.]
   (Biol.)
   The investing portion, or spherical envelope, surrounding the
   eccentric germinal spot of the germinal vesicle.
   [1913 Webster]