98 NICHOLS— SPERMATOGENESIS ONISCUS ASELLUS LINN. [April 4, 



of which the nucleus runs as a thread. They are stated to increase 

 in size through the assimilation of a secretion of the vas deferens. 

 They then become spirally twisted while in a certain limited section 

 of the vas deferens, presumably by a motion of their own. This is 

 said to be caused by a fibre running spirally the length of the 

 spermatozoon. The mature spermatozoon has the spirally twisted 

 structure of a rope of tow. It contains a twisted central fibre, not 

 visible externally, and the entire structure is surrounded by a 

 hyaline sheath. The spermatozoa are nearly motionless while in 

 the body of the male, but become extremely active in the recep- 

 taculum seminis of the female. This is said to be due to the loss 

 of the hyaline sheath. The curious fact is noted that the sperma- 

 tozoa coming from the right side of the animal are twisted to the 

 left and vice versa. 



1889. Mliller discovered in the spermatid of Ostracoda one or 

 two '' Nebenkerne." These form a *' Schwanzstuck " which s^rows 

 very long and is of complicated structure. Through the middle of 

 the tail runs the central fibre, at or near one end of which the 

 nucleus is located. The spiral twisting is referred to the contrac- 

 tion of the middle one of the three threads which surround the 

 central fibre. He does not agree with the opinion of Stuhlmann 

 concerning the inhibitive function of the sheath while in the body 

 of the male. 



Phyllopoda. 



1885. Zacharias describes the results of his observations and ex- 

 periments on the spermatozoa of the Phyllopod, Polyphemus, which 

 he shows to be capable of amoeboid movements. 



b. Commentary. 



The Crustacea as a class show an astonishing variety in the form 

 of the male reproductive elements. Knowledge of their intimate 

 structure is of course at present too incomplete to enable us to discuss 

 at any great length the homologies existing between them. But a 

 rough classification of them according to their external appearance 

 would place the bell-shaped or vesicular form characteristic of the 

 Decapods in one group and the form found in the Isopods, Gam- 

 marus, Mysis and Balanus, with more or less elongated nucleus and 

 tail of varying length, in another. The extremely peculiar form of 

 the spermatozoon of the Ostracoda might perhaps be referred to the 



