80 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



plasm surrounding the head region. Whether this is true of the sper- 

 matozoon after it has heen set free in the vas deferens, or whether this 

 envelope is reduced to a thinner layer, I cannot state, but the latter would 

 seem to be necessary for its active swimming and for its penetration of 

 the egg at the time of fertilization. However, in the latest stages oc- 

 curring in the follicles of the testes this sheath of cytoplasm is present. 



IV- Discussion of Literature. 



1. Nuclear Structures. 

 A. The Karyosphere. 



I shall first discuss somewhat briefly the structures which in Protozoa 

 correspond to the karyosphere, and later the nucleolus-like structures 

 in the cells of Metazoa which present marked points of similarity to 

 this body. 



In Protozoa. — Calkins (:01, pp. 245-278), from his own work and 

 that of numerous other investigators upon the cell structure of the 

 Protozoa, lias arrived at conclusions concerning nucleolus-like structures 

 in this group which are of great importance not only in considering uni- 

 cellular animals, but also in studying the analogous structures in the 

 Metazoa. With regard to the true nucleolus Calkins has the following 

 to say (p. 253) : " A distinct plasmosome or true nucleolus comparable 

 to the analogous structure in Metazoa apparently exists in no case save 

 possibly in Actinosphaerium, and even here it is limited to a passing 

 phase during mitosis (Hertwig, '98). It is probable that the structures 

 which have been almost universally but erroneously called nucleoli, do 

 not belong at all to this category of nuclear elements, but represent 

 either the functional chromatin which is aggregated into a central mass 

 (karyosome) during the quiescent or vegetative period of cell life, or the 

 intranuclear division centre." 



The lower types of nuclei often show considerable resemblance to 

 structures in the metazoan cell which have been called nucleoli or 

 karyosomes. Within the more differentiated nuclei of Protozoa are 

 often found masses of chromatin and achromatin which are nearly 

 identical with some of the "nucleoli" found in Metazoa, and at the 

 same time are strictly analogous to the nuclei of less modified Pro- 

 tozoa. The nuclei of primitive Protozoa more nearly resemble in struc- 

 ture and general behavior the karyosomes of many Metazoa and higher 

 Protozoa than they do the nuclei of these niox-e highly differentiated 

 cells. 



