32 



EMBRYOLOGY. 



known in earlier times under the name of direction bodies, or 

 polar cells. They have acquired the latter name because, in the case- 

 of eggs in which an animal pole is to be distinguished, they always 

 arise at that polo. After the conclusion of the second process of 

 budding, one half of the spindle, the other half of which was employed 

 in the formation of the second polar cell, is left in the cortical layer 



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IV. V. VI. 



Fig. 13. Formation of the polar cells in Asterias glacialis. 



In figure 7. the polar spindle ($p) has advanced to the surface of the egg. In figure II. there has- 

 been formed a small elevation (rk l ), which receives a half of the spindle. In figure ///. the 

 elevation is constricted off, forming a polar cell (ri 1 ). Out of the remaining half of the 

 previous spindle a second complete spindle (sp) has arisen. In figure IV. there bulges forth 

 beneath the first polar cell a second elevation, which in figure V. has become constricted off 

 as the second polar cell (r/t*). Out of the remainder of the spindle is developed (figure VI.) 

 the egg-nucleus (ek~). 



of the yolk (fig. 13 V and VI ek). From this arises a new, small, 

 vesicular nucleus, which consists of a homogeneous, tolerably fluid 

 substance without distinctly segregated nucleoli, and attains a 

 diameter of about 13 /*. From the place of its formation it usually 

 migrates slowly back again toward the middle of the egg (fig. 14 ek)^ 

 The nucleus of the mature egg (fig. 14 ek) has been designated by 

 me as Egg- nucleus, by VAN BENEDEN as female pronucleus. It is not 

 to be confounded with the germinative vesicle of the unfertilised egg. 

 Compare the figures of the immature egg (fig. 15) and the mature- 

 egg (fig. 14) of an Echinoderm, both of which are drawn with the 

 same magnification. The germinative vesicle is of very considerable 

 size, the egg-nucleus remarkably small : in the case of the former 

 one distinguishes a clearly developed nuclear membrane, a nuclear 

 network, and a nucleolus ; the latter is almost homogeneous, without 



