30 AETHUR DENDT. 



pressa "may be seen to be hanging, pear-shaped^ upon the 

 surface of the excretory canals, where it remains for a certain 

 time locomotive, until, after further development, it becomes 

 permanently fixed, and the locomotive envelope seems to pass 

 into a capsule." It is obvious that by '' excretory canals " 

 Mr. Carter means the inhalant canals, and he is probably 

 wrong in considering that the locomotive envelope of the ovum 

 passes into a capsule ; but the main fact, that the ovum at a 

 certain stage of its existence hangs freely from the surface of 

 a portion of the canal system, is clearly brought out. 



In Grantia labyrinthica I have distinctly observed the 

 ova hanging from the epithelial lining of the inhalant canals 

 by means of short peduncles, and projecting freely into the 

 lumen of the canal, where they must be washed by the incoming 

 stream of water. Fig. 35 represents an amoeboid ovum 

 approaching the surface of an inhalant canal ; and figs. 36 

 and 37 represent it, after having passed through the epithe- 

 lium, hanging freely by its short peduncle, awaiting ferti- 

 lisation. In fig. 37 the nucleus of the ovum appears very 

 near the surface, in a position suggestive of the formation of 

 a polar body. Probably the ova pass through the epithelium 

 of the inhalant canals in the same way that the white blood- 

 corpuscles pierce the walls of the capillaries in higher animals. 



After fertilisation the ovum probably migrates back into the 

 gelatinous ground-substance, and takes up its position near the 

 wall of a flagellated chamber, there to undergo the earlier stages 

 of its development. This seems more probable, from what we 

 know (10) of the position of the embryos, than the supposition 

 that it remains and develops in the spot where it is fertilised. 



It is probably a general rule in sponges that the ova are 

 fertilised while hanging from the walls of the canal system, 

 and that they migrate first of all through the canal-wall to be 

 fertilised, and then back again into the gelatinous ground- 

 substance to undergo development ; hence the necessity for 

 the amoeboid movements so characteristic of sponge ova. Thus 

 "we see that the sponge ovum plays an unusually active part in 

 the process of fertilisation, as it were meeting the spermatozoon 



