EMBRYONIC FISSION IN CYCLOSTOMATOUS POLYZOA. 203 



between the form of the adult ovicell and that of the younger 

 stages of the same organ. 



The ovicell is developed at the growing point, and it is here 

 that the early stages in the development of the egg take place. 

 A young internode^ may be described as an acute-angled 

 isosceles triangle with two sub-equal sides (AB, AC). Within 

 the triangle a calcareous septum occurs parallel to -45, cutting 

 oflF the oldest zooecium of the internode from the others. 

 Tho next septum is parallel to AC, and is nearer to the base 

 of the triangle. The formation of septa, alternately parallel 

 to AB and AC, gives rise to a series of alternate zooecia, an 

 arrangement characteristic of Crisia. The oldest zooecia are, 

 of course, those nearest to the apex of the triangle, and the 

 central part of the base is the region from which, with con- 

 tinued growth, fresh zooecia are cut oflp. It remains to be 

 stated that the growing-point, like the adult internode, is 

 flattened, and that the openings of the zooecia are lateral, and 

 are directed towards one of the flat surfaces of the branch. 



As the internode elongates, its proximal zooecia acquire their 

 full length, and cease to take part in the formation of the 

 growing-point. Or, explaining this by the former illustration, 

 let the internode grow to twice its former length, the growing 

 point remaining of the same width throughout its growth. 

 By producing the lines of the septa already present it will be 

 seen that if the growing point does not grow wider the older 

 zooecia will be excluded from it, their growth being completed. 

 It follows that the zooecia, several of which occur in a young 

 state at the end of the branch, become successively shifted to 

 the edges of the growing-point, preparatory to leaving it 

 altogether. 



The growth of the zooecia and of the ovicells takes place by 

 the apposition of fresh material at the distal end. The proxi- 

 mal end of each unit of the colony is first laid down, and the 

 last-formed portion is the aperture. Thus, by drawing a line 

 transversely at any level across an internode, whether the in- 

 ternode bears an ovicell or not, we obtain an accurate idea of 

 » Compare pi. XXIV, fig. 15. 



