144. _The Sphærastrum-Stage._--The _Actinophrys_-stage of the young Radiolaria, which proceeds immediately from the flagellate zoospore, is probably connected with the _Actissa_-stage by an intermediate form, which may be regarded as a simple skeletonless _Heliozoon_ with a jelly-veil; a well-known example of such a form is _Sphærastrum_ (in the solitary, not the social condition) and _Heterophrys_. This important intermediate form has arisen from the simple _Actinophrys_-stage by the excretion of an external structureless jelly-veil, such as is formed in many other Protista (_e.g._, in the encystation of many Infusoria). The young Radiolarian in this second _Heliozoon_-stage becomes a simple cell with pseudopodia radiating on all sides; its body consists of three concentric spheres, the central nucleus, the protoplasmic body proper, and the surrounding calymma or jelly-veil. When a firm membrane is developed between the last two spheres this _Sphærastrum_-stage passes over into the _Actissa_. The gap in our empirical knowledge which still exists between the flagellate stage (§ 142) and the simplest Radiolarian stage (_Actissa_, § 145), can be filled hypothetically only by the assumption of several _Heliozoon_-stages following one upon another. It is possible also that the capsule-membrane is not formed between the endoplasm and exoplasm (as here supposed), but that the membrane was formed first outside the cell and the extracapsulum subsequently secreted around it. 145. _The Actissa-Stage._--The first SPUMELLARIAN genus, _Actissa_, is not only the simplest form actually observed among the Radiolaria, and the true prototype of the whole class, but also the simplest form under which the Radiolarian organisation can be conceived. It is therefore extremely probably that _Actissa_ not only forms the common stem-form of the whole class in a phylogenetic sense, but is also its common ontogenetic or germinal form. Probably in all Radiolaria the _Sphærastrum_-stage develops immediately into the typical _Actissa_-stage, by the formation of a firm membrane between the protoplasmic body of the spherical Heliozoan cell and its jelly-veil. Thus arises the characteristic central capsule, which is wanting in the nearly related Heliozoa. It is further probable that all Radiolaria in their early stage will so far conform to the state of things in _Actissa_ as to have the capsule-membrane of the spherical skeletonless cell perforated everywhere by fine pores. This structure is retained in all SPUMELLARIA, whilst in the other three legions those structural relations of the capsule which are characteristic of each develop from the _Actissa_-stage.