Here Baer referred to Kolliker's paper. Since Kb'lliker 

 did not doubt the complete disappearance of the embryonic 

 vesicle, Baer, referring to his authority, left his own 

 completely accurate observations in doubt and considered it 

 necessary to check them again. Baer noted, however, that 

 he could never ascertain the moment when the nucleus was 

 completely absent. And what is more, he expressed confidence 

 that in the ova of frogs the embryonic vesicle would be 

 detectable in this period when, as it was believed, it 

 completely disappears. 



Further on, Baer passed to the description of the 

 following stages of segmentation. Soon after the division 

 of the ovum into two parts, "each nucleus begins by the way 

 previously described to form processes, and changes by 

 dividing in the middle into two new nuclei; the mass of yolk 

 adjacent to it also is divided, so that all the ovum dis- 

 integrates into four masses. Each quarter becomes rounded, 

 and in the center between them an empty space 16 is formed. 

 The division of quadrants takes place completely similarly; 

 moreover, the newly formed processes are situated at right 

 angles to the previous ones. This continues also during 

 the following divisions; moreover, for every new fragment 

 of yolk a nucleus is formed by division of the earlier one, 



Baer noted that the nucleus in the process of division 

 is not always distinctly demarcated from the surrounding 

 substance, "In the period of dormancy," Baer wrote, "a clear 

 boundary line is seen under the microscope, but at the time 

 of formation of the processes it cannot be observed with 

 definition" (p. 239). Further, he drew from his observations 

 another essential conclusion, concerning the method of 

 formation of cells in the process of early embryonic develop- 

 ment in the sea urchin. With great accuracy it can be 

 established, and for subsequent divisions also, that when 



15. A. Kolliker, "Beitrage zur Entwickelungsgeschichte 

 wirbelloser Thiere. 1: Uber die ersten Vorgange im 

 befruchteten Ei," ARCH. ANAT . , PHYSIOL. (1843), 

 pp. 68-141. 



16. This rudiment of the division cavity (blastocoel) is 

 called Baer's cavity by R. Remak. 



470 



