CYTOLOGY 167 



forms and making extracts from published 

 records. All animals are related together by 

 the continuity of the germ-plasm ; the remoter 

 the relationship, the closer the convergence may 

 be, and vice versa. An instance of nuclear con- 

 vergence has been noted recently by Minchin. 1 

 In the collar-cells of some calcareous sponges 

 (Clathrinidae) he found that the nucleus occupies 

 a position at the base of the cell, and the 

 flagellum arises independently from a granule 

 or blepharoplast situated at the surface of the 

 cell in the centre of the area enclosed by the 

 collar. In the Leucosoleniidse the nucleus 

 occupies an apical position and the flagellum 

 appears as a direct continuation of the pointed 

 end of the nucleus. 



Minchin quotes a parallelism to these alter- 

 native positions of the nucleus in the case of two 

 species of Mastigina described by Goldschmidt 

 (1907). Such a character, adds Minchin, in the 

 case of sponges, can have but little importance 

 in the struggle for existence, and yet in his 

 opinion it indicates the deepest phylogenetic 

 divergence in the pedigree of the calcareous 

 sponges. 



Cytological convergence, as between Metazoa 

 and Protozoa, yields many points of instructive 



1 E. A. Minchin, "The Relation of the Flagellum to the Nucleus 

 in the Collar-Cells of Calcareous Sponges." Zool. Anz., xxxv., 1909, 

 p. 227. 



