SPONGES 6 1 



adventitious envelope or follicle derived from the cells of the parenchyma 

 (Fig. 55, j). In Sponfjilla also the covering cells tend to disappear and to 

 be replaced by a similar adventitious follicle, which in this case may, 

 however, enclose several sperm balls. 



Nothing has as yet been made out with reference to the interesting 

 phenomena of chromosome reduction, now so universally established in 

 other animals. To judge from Fiedler's figures the number of chromo- 

 somes is small in Spongilla, apparently four in the germ cells and eight 

 in the somatic cells (?). The detailed structure of the spermatozoa also 

 remains to be studied. 



The oogenesis and the maturation of the ovum has been studied 

 in Spongilla by Fiedler, and more recently in Sijcon by Maas [15]. As 

 in other cases the history of the ovum may be divided into two periods — 

 tlie first of growth, the second of maturation. The ova are formed in all 

 parts of the body by growth of wandering cells. 



In Spongilla each ovum becomes surrounded by a follicle formed of 

 cells of the parenchyma, amongst which a certain number of trophocytes 

 work their way. The trophocytes are concerned with the nutrition of 

 the ovum ; it is remarkable, however, that the granules in the two 

 kinds of cells have different reactions, the nutriment received from the 

 trophocytes being worked up by the ovum into yolk granules, which 

 stain with bleu de Lyon in the way characteristic of such granules. When 

 the ovum is full fed no more trophocytes are to be seen in the follicle, 

 which by the growth and pressure of the ovum has assumed an endo- 

 thelial character. The full-grown oocyte has a large germinal vesicle 

 containing a large central mass of chromatin or nuclear corpuscle. 



During the maturation period the chromatin becomes concentrated 

 and individualised into chromosomes. Two polar bodies are given off in 

 the usual way.^ 



The fertilisation has been studied only by Maas in Sycon. ,The 

 spermatozoon penetrates the ovum before formation of the second polar 

 body. The two pronuclei swell up and come together at the middle of 

 the long axis of the ovum. They then break up to form the first 

 segmentation spindle, in which maternal and paternal chromosomes can be 

 recognised side by side, and distinct fi'om one another. The axis of the 

 spindle coincides with the longitudinal axis of the ovum. All subsequent 

 cleavages of the ovum are preceded by typical mitoses. 



* So Maas ; Fiedler, on the other baud, describes the formation of the polar 

 bodies, as well as the cleavage of the ovum, as taking place by means of a peculiar 

 kind of direct division. 



