No. 3] THE PROTOZOA AND METAZOA. 73 1 



in Noctihica are formed from the substance of the sphere, or 

 archoplasm. The mantle-fibers are probably nuclear in origin, 

 as indicated by Ishikawa. They are focused in the centrosome 

 and connect with the chromosomes. The microsomes compos- 

 ing them are at first only loosely strung together (Fig. 38), but 

 later they become more closely packed, forming solid fibers (Figs. 

 30-39). Since the chromosomes lie in a ring around the cen- 

 tral-spindle the mantle-fibers also surround it, and thus complete 

 the similarity to some mitotic figures in Metazoa (Figs, i, 28, 

 and 29). 



When fully formed, the mantle-fibers seem to be stiff fila- 

 ments of constant length and thickness. They do not shorten 

 as mitosis progresses, nor do they become thicker, but in the 

 telophase they appear the same as in the earlier anaphase 

 (Fig. 24). 



5. The Centrosome. 



It is with some hesitation that I undertake a description of 

 the centrosomes in N^octi/iiea, for they are so minute and are 

 accompanied by so many large cytoplasmic granules that, except 

 in mitosis, their identification is not only difficult, but sometimes 

 impossible. I am certain that Ishikawa, in many cases at least, 

 mistook some of the cytoplasmic granules for centrosomes, for 

 I have been entirely unable to find centrosomes of the size and 

 form described by him. Nevertheless, his general conclusion 

 that a centrosome in Noctihica does exist is correct ; for its 

 identification when connected with the mantle-fibers presents 

 no difficulty whatever (Figs. 23, 24, 28, and 38). In regard to 

 its origin Ishikawa had little to say, but he surmised that, in 

 some cases at least, it comes from the nucleus. 



While fully appreciating the difficulties in the way of deter- 

 mining the origin of a body so minute and so easily confounded 

 with other granules, there are, nevertheless, several phenomena 

 connected with the prophase of nuclear activity which give, I 

 believe, an apparent basis for the conclusion that the centro- 

 some is a minute granule lying inside the nucleus of the resting 

 cell, and that it migrates out into the attraction-sphere during 

 division. 



