254 FIELD. [Vol. XI. 



Prenant (45) finds that in the Reptiles it forms a cap to the 

 head ("la coiffe cephalique "), but that in the Pulmonates it is 

 dissolved in the cytoplasm which finally forms the tail of the 

 spermatozoon. 



From the fact that the " Nebenkern " does not in all species 

 act in exactly the same manner in the building up of the sper- 

 matozoon, but apparently sometimes the major part becomes 

 placed anteriorly, sometimes posteriorly to the nucleus proper, 

 the earlier investigators have overlooked the smaller portion 

 and hence thought that the head alone or the middle piece 

 alone was formed from the " Nebenkern," i.e., that the mito- 

 some passed entirely into the head or into the middle piece as 

 the case might be. But Platner's observations upon Pulmo- 

 nates and these upon Echinoderms may put the question in a 

 clearer light. In many cases, e.g., in the Pulmonates accord- 

 ing to Platner, the " Nebenkern " comes to form a considerable 

 part of the spermatozoon anterior to the nucleus proper ; the 

 rest of the "Nebenkern" forms the middle piece. He found 

 that in the spermatid the " Nebenkern " is made up of two 

 portions, a part, the "mitosoma," svirrounding a much smaller 

 body, the " centrosoma." In the change of the spermatid into 

 the spermatozoon these two parts separate ; the centrosome 

 part takes a position anterior, but the mitosome a position pos- 

 terior to the nucleus {i.e., becomes the middle piece). Now in 

 the Echinoderm spermatid the centrosome is at no time con- 

 tained within the mitosome, but the ultimate arrangement of 

 these parts in the mature spermatozoon is the same as in the 

 Pulmonates. The fact, then, that not only the middle piece but 

 also a part of the head proper is formed from the " Nebenkern " 

 largely explains the cause of so many apparently disagreeing 

 results. 



This comparison of the conditions obtaining in different 

 groups in regard to the "Nebenkern," and the relations of 

 mitosome and centrosome, together with the fact that mito- 

 some and centrosome throughout their history have identical 

 micro-chemical reactions (compare Figs. 14 to 28), has led me to 

 infer that they are differentiations of the same substance, yet 

 so well specialized that they subserve specifically distinct func- 



