32 HISTORY OF THE EGG FROM FERTILIZATION TO CLEAVAGE. 



not the idea been advanced by a well-known authority, it would not be 

 noticed here. When we reflect that, according to this view, we ought to 

 find caryokinetic division in the higher, and direct division prevailing in 

 the lower forms of life, we see how utterly indefensible it is. Take the 

 development of one of the higher animals, and you find the refutation 

 again complete. In the egg, the lowest stage, we find typical caryoki- 

 nesis ; the alleged direct division appears later, usually after the cells 

 have become small and observation difficult. Evidently the idea has no 

 sound basis, and here we will drop it. 



We have yet to learn pecisely what are the essential features of the 

 caryokinetic phases. Allowing that the centrosome is the leading factor, 

 it is easy to see that the astral rays might vary in intensity of expression, 

 according to the quantity and quality of the cytoplasm. If the centro- 

 some were present, and went through the regular course of division, we 

 should be justified in assuming that the influences at work were the 

 same, whether the rays were visible or not. At the outer pole of the 

 first spindle we could not detect any decided astral rays, while such rays 

 were observed around the deeper pole. The influences operating at the 

 two poles are presumably alike, but the conditions necessary for the sharp 

 definition of these rays are more perfectly represented around the inner 

 than the outer pole, — and herein lies the wdiole explanation of the differ- 

 ence. In some cases of supposed direct division, there may be a typical 

 centrosome division, and the process be so disguised by the constitution 

 of the nucleus or cytoplasm, or both, as to escape detection. The point 

 to be emphasized here is, that we have no conclusive proof of the exist- 

 ence of '• direct division," and that indirect division is not to be denied 

 on the ground that no astral or spindle rays are recognizable. All disputes 

 about the name will readily adjust themselves when the main question 

 has been settled. 



In the poles of the amphiaster, we have two bodies to distinguish ; 

 (1) the centrosome (Boveri), and (2) the clear sphere, from which the astral 

 rays depart. As we do not know that the latter is a " sphere of attrac- 

 tion " (Van Beneden), and as w r e are unable to determine precisely what 

 and how much can be identified with the archoplusm (Boveri) of Ascaris, 

 we follow Vejdovsky and call it periplast. 



