No. 2.] THE ECHINODERM SPERMATOZOON. 237 



those forms which live in the deeper water, e.g.^ most of the 

 spatangoids found at Naples, and particularly in Dorocidaris 

 papillata. In many specimens the entire reproductive gland 

 had become a mass of these "yellow cells." The presence of 

 these yellow cells suggests many interesting questions. It is 

 apparently a pathological condition and not symbiotic in the 

 true sense. It is remarkable that here shut in within the 

 urchin's shell they should find conditions favorable for growth. 

 The shape of the sexual glands in the different classes varies 

 very considerably as a comparison of PI. XV, Figs. 6-1 1, will 

 show. The variations are coordinated with the shape of the 

 body cavity in the different species, but the same general plan 

 is found usually throughout the class. So that the figure given 

 may be regarded as more or less typical for each group. 



Terminology. 



I have made use of the terminology, introduced by LaValette 

 St. George now most commonly adopted. The term sperma- 

 togone = " sperm mother cell " is applied to the original cell 

 which detaches itself from the germinal epithelium. The 

 spermatid is the cell which changes directly into the sperma- 

 tozoon. The one or more generations of cells which mediate 

 between the spermatogone and the spermatid are the sperma- 

 tocytes. As constituent parts of the cell the term cytoplasm 

 is used for the protoplasm in distinction from the nucleus : 

 nuclein = chromatin ; caryoplasma or caryolymph ^ nuclear sap. 



In view of the fact that the term " Nebenkern " has come to 

 be applied to many sorts of intracellular structures including 

 those whose morphology and physiology is known, it seems 

 best, since the history of the middle piece or " Nebenkern " is 

 now better understood, that this non-committal term " Neben- 

 kern " should be replaced by some term which gives a hint 

 as to the nature of this body. Since the middle piece or 

 " Nebenkern " of the echinoderm spermatozoon is formed from 

 the mitotic spindle, the term "mitosome," introduced by Plat- 

 ner, has been adopted and will be used to designate the middle 

 piece, = Nebenkern = corpuscle accessoire of other writers. 



