238 bulletin: museum of compakative zoology. 



drawing (Figure 15) is the position it had when the sperm-head sep- 

 arated from it. In this series of sections, and in another of mine simi- 

 lar to it, the base of the sperm-head is turned toward the egg-nucleus ; 

 in the section represented in the drawing (Figure 15) the turning has 

 been through an arc of 90 degrees. The fact that in both series the 

 base is turned toward the egg-aster shows that the force which caused 

 the turning must have acted at the base. The difficulty comes now in 

 finding the structure in which that force resides. I have said that I 

 have been unable to find a centrosome in the series represented in 

 Figure 14, and I am not certain of having seen it even in the prepara- 

 tion from which Figure 15 was drawn. However, from what we know 

 of the turning of the sperm-head in other forms, we can assume that 

 the centrosome exists in this case, even if it is not visible. I have 

 shown in the figure two structures which may be centrosomes. Extend- 

 ing from the base of the sperm-head in advance and to the left of it I 

 have shown two fibres, thicker than the others that surround the sperm- 

 head, each containing at its middle point a minute granule. On exami- 

 nation of the specimen with a T ' g inch homogeneous Zeiss lens these two 

 fibres are seen to be composed of numerous very fine fibrils. This recalls 

 the condition that Kostanecki ('96, Figurel) shows for the free sperm-head 

 of the sea-urchin. In the sea-urchin, however, there is only one bundle 

 of rays, and that extends from the base directly toward the centre of 

 the sperm-aster. In the other series (not figured) of the same stage as 

 the one represented in Figure 15, there is evidence of a granule in a 

 bundle of fine fibres which extends from the base of the sperm-head. 

 Fibres can also be seen radiating from this granule into the surround- 

 ing vacuole, but these radiations have no connection, as far as I can 

 determine, with the more prominent threads extending from the sperm- 

 head through the vacuole. Since in Figure 15 — a stage in which 

 the centrosome is known to be active — there is no evidence of connec- 

 tion between a centrosome and the fibres radiating from the sperm-head, 

 the radiating threads about the sperm-head shown in Figure 14 can 

 hardly be said to be due to the influence of a centrosome. 



The evidence I have of the existence in the egg of a structure which 

 I can say positively is a sperm-centrosome in its earliest stages is very 

 meagre. I have seen in the egg many appearances in close proximity 

 to the free sperm-head which suggested an aster, but examination in 

 other parts of the cell afforded many examples of similar ill-defined 

 aster-like structures. In Figure 211 have drawn a structure that may 

 be the sperm-aster in an early condition; it lies within the vacuole of 



