34 HISTORY OF THE EGG FROM FERTILIZATION TO CLEAVAGE. 



has become indistinct. The astral figure consists of a minute, clear 

 circular area, which is the centre of a feeble radiation. The clear centres 

 appear to be in actual contact with the poles of the nucleus, and no rays 

 are visible on the side of contact. 



In the clear central area of the aster, we often find a body that ap- 

 pears to be perfectly homogeneous, and only barely differentiated by a 

 dark hue imparted by osmic acid. The action of the preservative fluids 

 varies somewhat, and the demonstration of this body is frequently missed. 

 The close contact between this body and the nucleus, together with the 

 early effacement of outlines at the poles of the nucleus, led to the suppo- 

 sition that it was a nuclear derivative. Our observations were completed 

 and our plates finished early in 1886, long before the history of the cen- 

 trosome had been made known. We have no hesitation now in identify- 

 ing this structure with the centrosome, and are quite ready to concede 

 that its origin from the nucleus is rendered uncertain, if not disproved, 

 by the studies of Boveri. In preparations mounted in Mo (Figs. 33, 

 34), we are rarely able to distinguish the centrosome ; but in sections 

 (Fig. 32), we seldom fail to detect it. Distinct outlines are, however, in 

 most cases absent, and hence the appearance strongly suggests an outflow 

 of nucleoplasm at the poles. Boveri's observations are more decisive, and 

 render it probable that the centrosome is of cytoplasmic origin. 



In later stages of cleavage, we have obtained much more distinct 

 views of the astral centre (Figs. 3, 4, PI. XXIX.). In Figure 3, Plate 

 XXflll., it is a sharply outlined area, but it is doubtful if this can be 

 considered the centrosome. In Figure 4 of the same plate, the centro- 

 somes are slightly stained, but not definitely outlined. 



In most of our preparations, we are able to distinguish in the poles 

 of the amphiaster, not only the centrosome, but also a clear sphere con- 

 centric with it, and it is in the periphery of the latter that the astral rays 

 end. This clear sphere is what Van Beneden and Neyt have called the 

 sphere of attraction ; Vejdovsky, periplast ; Boveri, archophsm. Van Bene- 

 den regards this sphere as an essential element of every cell, and in this 

 opinion Rabl 27 concurs. 



For reasons before stated, we will employ Vejdovsky 's term for this 

 body. Its precise relations with the centrosome have not yet been made 

 clear, and our studies do not throw any light on this point. 



27 C. Rabl. Ucber Zellteilung. Anat. Anz., IV., No. 1, Jan. 10, 18S9, p. 21. 



