616 W. J. SOLLAS. 



of support and as a store of nourishment in reserve. The 

 differentiation would now become permanent and stable, both 

 the cytoblastic and the flagellate cells finding themselves 

 advantaged. In this state the compound infusorian might 

 have existed for a considerable interval, reproducing itself by 

 the subdivision of the cytoblastic cells into flagellate offspring. 

 The work of reproduction in course of time devolving upon 

 some of the better-fed cytoblastic cells, instead of being per- 

 formed by all : the Infusoria, resulting from the metamorphosis 

 of each ovum, might in time become more numerous, and 

 remaining aggregated produce a free-swimming globular cluster 

 of Infusoria. 



It is in every way probable that several such agamic ova 

 would be present in a single colony at the same time, and that 

 they would coexist in various stages of maturity. Eventually 

 one may suppose that the young flagellate cells arising from the 

 division of one ovum bored their way into the vicinity of 

 another ovum, mature, but not yet in process of division, and 

 penetrating its substance, conferred upon it that advantage 

 which results from fertilisation. It is interesting in this con- 

 nection to find that the young infusorian produced by a 

 choano-flagellate parent is at first without a collar and con- 

 tractile vesicle, and of much smaller size than the adult. So 

 similar is it apparently to a Spermatozoon that S. Kent was 

 led by this similarity to throw quite unjustifiable doubts on the 

 real existence of spermatozoa in the Sponges. 



The ovum fertilised by the young flagellate cell (ancestral 

 form of Spermatozoon) might very naturally, owing to the 

 increased energy conferred upon it, have proceeded farther with 

 its metamorphosis while still within the parental tissues 

 than had previously been the case. If at the same time it 

 continued to receive nourishment from the parent stock it 

 might be some time before segmentation was carried far 

 enough to produce cells small enough for metamorphosis into 

 flagellate collared cells, and in order to secure to each cell of 

 the dividing mass its fair share of nourishment it is possible 

 that each may have worked its way to the exterior, so as to 



