266 MONTGOMERY. [Vol. XV. 



PAGE 



19. Gland cells of Piscicola rapax (Verr.) 483 



20. Mesenchym Cells of Cerebratulus lacteus (Verr.) 494 



21. Ganglion Cells of Nemerteans 49^ 



IV. General Comparisons and Conclusions 497 



Appendix to the Literature Reviews 539 



Literature List 542 



Explanation of Plates 561 



L INTRODUCTION. 



The following studies are based upon animal cells, both egg 

 cells and somatic cells having been investigated. They were 

 made, primarily, with a regard to the morphology of the true 

 nucleoli (plasmosomes), though numerous other points in onto- 

 genetic cellular development have been considered. In con- 

 nection with these observations the zoological literature upon 

 the subject of nucleoli has been reviewed as thoroughly as 

 possible, and, less completely, the literature from the botan- 

 ical standpoint as well ; reviews are given of these observa- 

 tions of previous writers. No attempt has been made to 

 review the literature from the pathological standpoint. Under 

 the caption "General Comparisons and Conclusions" are com- 

 pared together the more important deductions from my own 

 observations, and these are compared with those of previous 

 investigators. 



The nucleoli are cellular structures which have been studied 

 to much less extent than other constituents of the cell, and 

 though there are numerous observations upon them, these are 

 so scattered through works of more general import that it is 

 well-nigh impossible to collect together all the previous inves- 

 tigations upon the subject. I hope that this explanation may 

 be taken as an apology by any authors whose papers I have 

 chanced to overlook. 



At the laboratory of the Fish Commission at Woods Holl, 

 the following species were collected by me : Montagua, Amphi- 

 j>orus glutinosits, Tetrasteninia catcnulatum, Zygoncmcrtcs, 

 Linens, Polydora, and Piscicola. At Sea Isle City, at the labo- 

 ratory of the University of Pennsylvania : Tetrastemma elcgmis, 

 Doto, and certain of the species found at the former locality. 

 Stichostemma was collected in the aquaria of the University 



