BIGELOW: NUCLEAR CYCLE OF GONIONEMUS MURBACHIL 295 



through the egg, is accompanied hy a controsome, which divides into 

 two, forming the centres of two separate asters ; and these the author 

 traces directly into the asters of the first cleavage spindle. 



Wulfert (: 02) has reached much the same conclusions in the case of 

 Gonothyrea except that he believes that a centrosome is to be seen 

 within the germinative vesicle before the breaking down of its mem- 

 brane, in spite of the fact that no such structures are present in the 

 ensuing maturation division. In addition to his observations on the 

 centrosome, Wulfert has followed the details of the union of the germ 

 nuclei, with results to be discussed fully later on. Harm (: 02), besides 

 confirming the work of the two students last mentioned, was able to 

 trace the history of the chromatin in the maturation divisions of Clava, 

 finding that in the first division there are about sixteen tetrads, while 

 in the second, although the number of chromosomes is the same, each 

 instead of being quadripartite is bipartite. 



Finally, two recent papers, one by Hargitt, the other by Hill, deserve 

 mention. Hargitt (: 04) maintains that in Peunaria there is an appar- 

 ent dissolution of the germinative vesicle at the time the egg is set free 

 — an apparent phenomenon described by many of the early observers in 

 coelenterates as well as in other groups, but one which has been shown 

 by 0. Hertwig ('78 b ), Fiedler ('88), and others, to be apparent rather than 

 real. Hill (: 05) goes even further, for he contends that in Alcyonium the 

 polar cells are formed by a direct division of the germinative vesicle, and 

 that thei'e follows a considerable period when the egg is actually, as well 

 as apparently, enucleate. But neither his material nor his figures seem 

 sufficient to establish the actual occurrence of such a remarkable type 

 of development. 



IV. Observations. 

 A. Mitosis in the Somatic Cells of Adult Tissues. 



In the adult Gonionernus somatic cells are so rarely found in active 

 mitosis that it has taken a long search to find an approximately com- 

 plete series of the stages. I had hoped to be able to study mitosis on 

 different tissues for the sake of comparison, but because of the rarity of 

 the process I have been unable to do so, and shall therefore limit the 

 description to the endoderm cells which line the radial canals. The few 

 mitotic figures found among entoderniic gland cells and ectoderm cells 

 agree in all essentials with those of the endodermal epithelium. 



These epithelial cells are of the ordinary undifferentiated type, the 



