ON THE DEVELOPMENT OF TUBULIFORA. 137 



really the thickened ed^je of the terminal membrane^ which is 

 deeply invaginated in the middle^ as was the case in the 

 earlier stage. A longitudinal section of the ovicell thus has 

 the form seen in fig. 22, in which the middle of the 

 terminal membrane is greatly depressed, so as to be widely 

 removed from the chitinous part of the membrane (ectocyst), 

 which is tightly stretched across the open part of the calcare- 

 ous funnel. I am doubtful how far this is normal. Some 

 amount of retraction of the living tissues from the orifices of 

 the zocecia almost certainly takes place when the animals are 

 killed, and it is possible that the protoplasmic terminal 

 membrane is normally in close contact with its chitinous 

 ectocyst. It is not easy otherwise to see how the ectocyst is 

 formed, unless the terminal membrane is capable of attaching 

 itself from time to time to the ectocyst, which, it must be 

 remembered, increases in extent so long as the ovicell con- 

 tinues to expand. 



Decalcification is a further source of alteration. Bubbles of 

 gas accumulate during this process in various parts, and are 

 responsible for a good deal of distortion of tlie protoplasmic 

 structures. The bubbles often find much difficulty in making 

 their way through or past the ectocyst, and an accumulation 

 of gas between the latter and the living part of the terminal 

 membrane may be responsible for a considerable amount of 

 depression of the former. As, however, the tissues were well 

 hardened before being decalcified, I think it probable that the 

 gas would rather tear the tissues than alter the entire position 

 of an epithelium which had lost its flexibility as the result of 

 long immersion in spirit. 



I have examined sections of nearly seventy ovicells in this 

 stage. About half that number belonged to T. plumosa, 

 eight to T. liliacea, and the remainder to T. phalange a. 



Fig. 19 is a longitudinal section of an ovicell of T. plumosa 

 at the beginning of stage E. The terminal membrane is 

 deeply invaginated in a neighbouring section. The distal part 

 of the ovicell is still nearly solid, and contains numerous 

 excretory vesicles. The vestibule is somewhat more distinct 



