446 SCIENCE PROGRESS. 



it presents us with a more primitive, less specialised con- 

 dition than we find in the Metazoa sensn sti'ictiori. 



The reproduction of sponges on the other hand is of a 

 distinctly Metazoan type, by means of true ova and sper- 

 matozoa. This is a fact which has been called in question 

 at various times, but, without entering into a detailed dis- 

 cussion, it must be regarded as established now beyond 

 the possibility of doubt, in view of the accurate observations 

 of Schulze upon various types of sponges, Fiedler (1888) 

 upon Spongilla, and many other authors. Their evidence 

 is in no whit invalidated by the fact that many of the 

 descriptions of supposed spermatozoa relate in all probability 

 merely to ordinary flagellated cells. On the other hand, 

 full and detailed accounts of the phenomena of the growth 

 and maturation of the ovum in different types are much to 

 be desired. The onlv such account existins^ is that driven 

 by Fiedler (1888) with reference to the ovum o{ Spongilla, 

 which shows some remarkable deviations from the type 

 usually considered as the normal for Metazoa, The polar 

 bodies, for instance, do not originate by a karyokinetic 

 division of the nucleus of the ovum, but are, as it were, 

 budded off after disappearance of the nuclear membrane 

 from the large spherical chromatin mass occupying the 

 centre of the vesicular nucleus. They arise, therefore, by a 

 process more akin to direct division, and a quite similar 

 process, only difTering in that the central chromatin mass 

 divides into equal halves within the still present nuclear 

 membrane, occurs at each division of the nucleus during 

 the segmentation of the ovum. The absence of karyokine- 

 sis in processes where its occurrence is usually so constant 

 is the more remarkable when it is seen that true and typical 

 mitoses are found in the tissue cells and the spermatogenesis 

 of SpoiLgilla. Not only are more observations upon other 

 forms much needed, but the details of the fertilisation of 

 the ovum are quite unknown in any sponge and remain for 

 description. 



Some authors have regarded the ovum of sponges as 

 an asexual reproductive body made up of a fusion of cells. 

 We have noticed already Kent's statement that the ovum 



