163 EDOUARD VAN BENEDEN. 



period between the yolk and the ovular membrane. The 

 opening enlarges, and the membrane of the vesicle becomes 

 gradually detached from its contents. Finally, the latter 

 are evacuated and the membrane spreads itself out on the 

 surface of the germ. Some observation of the same author 

 on " The Germinal Vesicle of the Fowl " furnish a remark- 

 able and complete confirmation of the conclusions at which 

 the illustrious von Baer arrived fifty years ago. 



Soon afterwards there appeared the beautiful researches of 

 Kleinenberg " On the Anatomy and Development of the 

 Fresh Water Hydra." Kleinenberg^ thus describes the mode 

 of disappearance of the germinal vesicle : — " About the time 

 when the production of pseudocells is completed the germinal 

 spot undergoes a retrograde metamorphosis. At first it loses 

 its circular outline and becomes irregular and angular; its 

 substance appears coagulated ; then it breaks up into little 

 fragments, and these, unless I am mistaken, finally dissolve. 

 So long as the egg was an amoebiform body, the germinal 

 vesicle was situated at the centre of the yolk ; but from the 

 time that the egg begins to become rounded it takes an 

 excentric position, and approaches that pole which is turned 

 towards the surface. It takes its situation near to the 

 surface, and is now covered only with a thin layer of plastic 

 material. Here it also begins to undergo a retrograde 

 metamorphosis which ends in its complete disappearance. 

 Its gi-anular contents become more and more liquefied ; a 

 part of these contents escape from the membrane with the 

 result that the latter, which had previously remained uni- 

 formly stretched, becomes collapsed so as to form a tube of 

 generally ovoid shape, the wall of which is thickened and 

 folded at certain points. The part of the contents which 

 has remained in the interior breaks up into isolated shining 

 bodies of rounded or angular form, and of very different 

 dimensions ; amongst them are scattered some drops of a 

 fatty liquid." 



Kleinenberg thinks that these bodies are composed of a 

 fatty material, or at least that they consist of the material 

 which results from the transformation of albuminoid sub- 

 stances, and which we observe in many pathological tissues, 

 where this appearance is a sign of fatty degeneration. 

 According to his view the germinal vesicle disappears by 

 fatty degeneration. On one occasion Kleinenberg believed 

 he observed an actual hole in the germinal vesicle. " ]f this 

 is a normal phenomenon," says Kleinenberg, " it is possible 

 that the contents of the vesicle escape and mix with the sur- 

 ' Kleinenberg, ' Hydra,' Leipzig, 1872, p. 42. 



