the nuclei are surrounded with a small layer of yolk the new 

 cells are not formed "inside the maternal ones" ("yolk bodies," 

 in Baer's terms) . 



Up to the stage of thirty-two blastomeres he could note 

 directly the appearance of new blastomeres by division. Later 

 on when they became numerous, it did not seem to be possible 

 to trace the appearance of each, "However, here and there," 

 Baer wrote, "in the yolk bodies, situated in the edge, the 

 processes of division can be seen also as earlier. Even when 

 the embryo leaves the ovum membrane and moves by the help of 

 the cilia, every granule or each histogenic element (cell) 

 possesses a very distinct nucleus" (p. 240), Baer could not 

 trace the subsequent development, due to the movement of the 

 embryo. Nevertheless, on the basis of the observations 

 described above he reached the conclusion that later the 

 histogenic elements (i.e. cells)result from earlier existing 

 ones by means of the same kind of division. The general 

 conclusion with which he ended the report stated: "The 

 division of the yolk is considered only the beginning of 

 the histogenetical separation, which continues uninterruptedly 

 to the final formation of the animal. If this presentation 

 is correct, then the question of the pre-existence of the new 

 individual, before fertilization, is beyond any doubt. The 

 unfertilized ovum is an embryo having latent life. The 

 fertilization makes its life active" (p. 240), 



This short paper of Baer's, as can be seen from its 

 summary, contains an extremely rich content. There is no 

 doubt that Baer saw with complete clarity all these processes 

 of maturation and division of the ovum, which can be seen by 

 intravital observations. He surpassed, by many decades, the 

 views of his contemporaries, showing, first, that the nucleus 

 of the fertilized ovum does not disappear, but becomes only 

 less distinctly visible. He saw further the appearance of 

 the radiant achromatic figures in the mitoses of division, the 

 change at the time of their division, the disappearance of 

 the nuclear membrane at the time of mitosis, and its appearance 

 in interkinesis. 



Drawings were not appended to the published report. If 

 one looks at a picture of the division of the sea urchin ovum 

 by intravital observation performed approximately a century 



471 



