AND THE LOWER ANIMALS. 17 



notwithstanding all the knowledge and skill ex- 

 hibited in their researches, have not always agreed; 

 and how the latter has occasionally observed ova 

 of an exceptional character. The slowness of the 

 changes which the yolk undergoes, and the ntter 

 impossibility of continuous observation, account 

 easily for these apparent contradictions. Most pro- 

 bably the phenomena are in reality identical.* In 

 Hermella and Teredo, the structure of the egg, 

 whether impregnated or not, is modified by this 

 agitation of the yolk. The germinal vesicle and the 

 germinal spot both disappear. Their contents become 

 mingled or blended, as it were, with the substance of 

 the yolk. Here the analogy we are demonstrating is 

 complete. In Mammalia, as in Mollusca, and Worms, 

 the distinction between the three vesicles is lost, 

 whether impregnation has taken place or not. In 

 the highest, as in the lowest member of the animal 

 scale, the egg thus exhibits its peculiar activity. 



If the egg of Hermella or Teredo has not been 

 impregnated, the movements of the yolk-mass increase 

 in rapidity, and become more and more irregular. 

 The egg at first becomes discoloured, and eventually 

 decomposes. In like manner, doubtless, the un- 



* Some of Messrs. Barry's and Bischoff 's drawings have forcibly 

 reminded me of the transitional appearances which I observed in 

 the eggs of Hermella. Thanks to the rapidity of the phenomena 

 in the latter, I have been able to see these appearances obliterated 

 and replaced by others. — (Annales des Sciences naturelles, 1848.) 

 I did not then attach any importance to this general fact. Deprived 

 of this advantage, my fellow-workers have been unable to examine 

 the matter as I have done. Moreover, since the publication of my 

 memoir on the development of Hermella, everything that I have 

 stated on the subject has been confirmed. 



C 



