44 DUBLIN 



a very different nature, since they arise by the opening up of the 

 angle of the primary V's to form the straight, bivalent, longitu- 

 dinally split, parallel rods which, by later uniting at both pairs 

 of ends form a ring, as in TJialassema or the Cope pod. Each 

 quadrant is, therefore, in Pedicellina, a chromatid (McClung), 

 and not one half of one as appears in the Amphibia from both 

 Flemming's and Montgomery's interpretation. It may there- 

 fore be said, that from the time of the straightening out of the 

 rods in the early oocytes, up to the first maturation, Pediccllina 

 parallels closely every essential process to be observed in the 

 chromosome formation in the Copepods. 



As has been previously observed, the rings are not limited in 

 their presence to the maturation divisions. At variance with 

 the assertion of Montgomery, '04, that the heterotypic mitoses 

 is always a reducing division, the rings in Pedicellina occur in 

 addition not only in the divisions of the primordial germ cells, 

 as Hacker found in the Copepods, but they occur in the soma- 

 tic tissues as well. It is clear that in these cells the rings are 

 of an entirely different constitution from those found in the 

 maturation divisions. In the former, they are only transient 

 structures which have arisen by the temporary connection of 

 the ends of the daughter V's, at the metaphase, while in the 

 latter they represent structures which have had a long history 

 in their formation and are essentially adaptations for the proper 

 equipment of the germ cells. It is therefore not the form but 

 the constitution of the rings that is of importance for the proper 

 interpretation of the nature of a division. 



{D) The Maturation Divisions. 



The problem of the maturation divisions is not at the present 

 time, in a very satisfactory state. On the one hand, in a con- 

 siderable number of forms of which Crepidula (Conklin, '02) 

 and Echinus (Bryce) are typical, the chromosomes are not 

 visible as such during the growth period of the Qgg, and appear 

 in very nearly their final form only shortly before the first ma- 

 turation division. In such cases it is obvious that the nature of 

 the division is full of uncertainty, and such indeed, has been the 



