No. 2.] THE ECHINODERM SPERMATOZOON. 25 1 



spermatogone. Study of some more favorable forms will 

 decide this. 



With the formation of the tail it first begins to be possible 

 to say which part of the spermatid will be the anterior point of 

 the spermatozoon. The centrosome, which hitherto apparently 

 has had no special position with reference to the nucleus (Figs. 

 34, 39), now comes to lie close beside it, and, except in the 

 Echinoidea, usually directly opposite to the point where the 

 tail is forming (Figs. 35, 36, 40, 42, 43). 



With the diminution of the cytoplasm around the nucleus, 

 the centrosome comes to lie in a depression in the nucleus, but 

 remains entirely outside of the nuclear membrane. This con- 

 dition is probably brought about mechanically by the pressure 

 of the tightly drawn cell-membrane which invests the sperma- 

 tozoon, pushing the unyielding refringent centrosome into the 

 wall of the nucleus. 



In the Asteroidea examined the centrosome consists of two 

 distinct parts or substances ; a clear, transparent, lightly staining 

 part surrounding a dumbbell-shaped body deeply staining with 

 dahlia (Figs. 14, 22, 41, 49, 51). This dumbbell-shaped body 

 reminds one of the figures given by various investigators for 

 the first stage in the division of the centrosome. 



As the table (p. 246) shows, the size of the centrosome varies 

 widely in the different classes, and even in the different species 

 within the class. It is largest in the Holothurioidea, and smallest 

 in the Echinoidea. Of the Echinoidea examined it is smallest 

 in Echinus microhibeiriilatiis (Fig. 46), where its diameter is 

 only about equal to the diameter of the tail of the spermato- 

 zoon. The very small size of the centrosome in the Echinoidea 

 is probably the reason why it seems to have been overlooked by 

 Pictet and others, who have worked on these forms alone. 



By means of those reagents which cause a slight swelling of 

 the nucleus or of the centrosome, notably chloride of manga- 

 nese, dilute acetic acid, etc., the centrosome can be made to 

 slip out of the little depression in the surface of the nucleus 

 (Fig. 20), though it is still held close to the nucleus by the 

 cell membrane which surrounds the entire spermatozoon. The 

 centrosome preserves its shape very well with most reagents. 



