456 EIOHARD EVANS. 



processes in the Metazoa." Mr. Miuchin finally concludes 

 that the "theory of the Metazoan nature of sponges offers in 

 the present stage of our knowledge fewer difficulties than the 

 theory of inde[)eadent descent from Choanofiagellata/' 



In the course of the few remarks added here I propose to 

 point out the difficulties attending a theory of the Metazoan 

 nature of sponges, and to lay stress on certain points which 

 favour the theory of their independent descent from the 

 Choanoflagellata. 



In the above quotation from the last paragraph of Mr. 

 Minchin's article three points are mentioned as the mainstays 

 of the Metazoan theory, namely, (a) sexual reproduction; (b) 

 segmentation of the ovum ; and (c) the formation of germ layers. 



Those who uphold the Metazoan theory of the nature of 

 sponges have strained every nerve to find some parallelism 

 between the development of sponges and that of the Metazoa, 

 while they have neglected to seek for points of comparison 

 between their development and certain processes which occur 

 in the multiplication or reproduction of the Protozoa, especially 

 in the higher and most differentiated members of the latter 

 sub-kingdom. 



We may consider in the first place the arguments derived 

 from sexual reproduction. Nothing more is meant by this 

 term than that two cells, specially developed, unite together to 

 form one cell, which is capable of giving rise to an animal 

 similar to those which produced the original cells, which by 

 their fusion became the cell in question, or fertilised egg-cell. 



In the Metazoa the development and maturation of the germ 

 cells are more or less uniform, allowing for certain variations, 

 when the processes are looked at from a theoretical point of view, 

 especially in the mode of reduction of the chromosomes. The 

 only account existing of the maturation of the ovum in 

 sponges is that of Fiedler (3), in which the ovum of Spongilla 

 is described as showing remarkable deviations from the type 

 usually considered as normal for the Metazoa. Perhaps the 

 most important peculiarity is the origin of the polar bodies 

 by a process more akin to direct division than to mitosis. 



