ON THE DEVELOPMENT OF TUBULIPORA. 145 



nuclei (8/^). These nuclei appear to be characteristic of the 

 growing tissue of the primary embryo, since they occur at a 

 part where no separation of secondary embryos is taking place. 

 "When traced to a part where a secondary embryo is being 

 constricted off, these large nuclei are seen to become smaller by 

 division, and to form the smaller nuclei (3*5 fx) of the inner 

 layer of the secondary embryos. Evidence of fission was not 

 obtained in all the old ovicells. 



The young secondary embryos are always embedded in 

 nutritive tissue, whatever the age of the ovicell. In many 

 cases the occurrence of a group of young secondary embryos 

 in close proximity to one another probably implies recent 

 embryonic fission. As development continues, the secondary 

 embryos become ciliated externally, and may then become 

 quite free in the cavity of the embryophore, the wall of which 

 may be reduced to a thin nucleated film of protoplasm in old 

 ovicells. 



The cavity of the ovicell is ultimately almost completely 

 filled by the embryophore, the lobes of which may be several 

 times divided. Even the axial lobe may give ofi" secondary 

 lobes (fig. 33), and it is clear that this must take place in 

 many cases from the external conformation of the ovicell. The 

 growing edge of the ovicell is usually conspicuous at the end 

 of any lobe which is still incomplete, but it disappears with 

 the complete closure of the lobe^by a calcareous wall. Some 

 of the older ovicells suggest as a possibility that the number of 

 excretory vesicles may be reduced in some way during stage 

 G, but others were provided with numerous vesicles. 



VI. The Morphology of the Internal Parts of the Ovicell. 



There can be little doubt that the embryophore of Tubu- 

 lipora corresponds in general with that of Crisia and 

 Lichenopora; but it is not easy to decide how far the homo- 

 logy is an exact one. 



In Crisia (15) the ovicell is at no period of its life an 

 ordinary zooecium. This would almost follow from the fact 

 that its proximal end has not the character of a simple, cylin- 



VOL. 41, PART 1. NEW SERIES. K 



