role which this nucleus (or embryonic spot) plays 

 in the ovum of the sea urchin occurs in other 

 animals in the embryonic vesicle. In addition to 

 this, in the ovum of the sea urchin the part called 

 embryonic vesicle disappears long before the end of 

 maturation. (p. 238) 



If the details of maturation remained unclear for Baer 

 during the study of the processes of segmentation he could, 

 despite his imperfect means of observation, see much and 

 essentially move ahead of his contemporaries. According to 

 Baer's description, after fertilization the ovum nucleus 

 greatly submerges in the yolk and after some minutes it seems 

 that it completely disappears. "However, under the micro- 

 scope," Baer state, "the nucleus may be seen, although its 

 boundaries are unclear due to uneven refraction of the 

 surrounding granular yolk, Sometimes only a limited light 

 radiance is seen. During the delivery of the ovum its 

 boundaries, having the form of a circle, become more distinct" 

 (p. 238). 



Baer described the process of division itself in the 

 following manner: 



After the period of dormancy, the nucleus, having 

 until then a spherical form, very quickly becomes 

 elongated, and at the same time processes of 

 protuberances appear from both of its sides. These 

 ends of the nucleus swell, and its center becomes 

 thin and is quickly torn off, so that two comet- 

 shaped nuclei with tails directed towards each 

 other are formed. The tail-shaped appendage quickly 

 extends inside its spherical mass, and then two 

 nuclei are apparent .... Just before division 

 the nucleus increases in size; at the time of 

 division this increase progresses, so that each of 

 the two new nuclei is approximately equal in size 

 to the initial one .... Only after that, when 

 both of the new nuclei are separated from each other, 

 does the re twisting of the ovum begin, in consequence 

 of which it is divided into two halves near to each 

 other, each surrounded by its portion of yolk, 

 (pp. 238-239) 



469 



