DUBLIN 



Preface. 



In the following study I have endeavored to follow out, as 

 fully as possible, the history of the germ cells in Pediccllhia 

 from the point of view that has been suggested by the recent 

 work of Montgomery, Sutton and Boveri. In line with the 

 work of the first two, my principal aim has been to examine 

 the reduction problem and especially the process of synapsis, 

 where, according to these authors, the reduction occurs. It is 

 due to them that the proper attention has been attracted to the 

 study of the earlier generations of the germ cells where the 

 beginnings of the processes that culminate in the later matura- 

 tion phenomena may be observed. Such a study, as far as the 

 material permitted, I have attempted to make, and what are 

 perhaps the most significant results of this study were thus 

 obtained. 



The conclusions of these authors involve as a fundamental 

 principle, the likeness of the processes in the history of the e^^g- 

 and the sperm-nuclei. Yet not since the work of Hertwig and 

 Boveri, more than a decade ago, has a work of any complete- 

 ness on both oogenesis and spermatogenesis, in the same ani- 

 mal, appeared. Indeed, with the exception of Ascaris and a 

 iQ.\N other forms, on which isolated observations have been 

 made, in no one species has the history of the chromatin been 

 studied throughout. For such a purpose Pedicellina ainericana 

 is in several respects very favorably adapted. While neither 

 the male nor the female germ-cells are in this form especially 

 large, all the stages in development from the larva to the ma- 

 ture polyp are available, making it possible to follow the pro- 

 cesses with comparative ease, from the very beginnings of the 

 differentiation of the germ-cells up to fertilization. Fertiliza- 

 tion and development of the embryo, moreover, occur within 

 the maternal brood pouch ; and this, coupled with the com- 

 paratively small size of the individual polyp, makes possible a 

 very direct comparison between the nuclear changes in the so- 



