REPORT ON THE RADIOLARIA. XCIU 



BIOGENETICAL SECTION. 



A SKETCH OF OUR KNOWLEDGE OF THE DEVELOPMENT OF THE RADIOLARIA 



IN THE YEAR 1884. 



Chapter V.— ONTOGENY OR INDIVIDUAL DEVELOPMENT. 



(§§ 141-152.) 



141. Individual Developmental Stages. — The germinal history of the Eadiolaria 

 presents great obstacles to direct observation, and hence is very incompletely known. 

 The fragmentary observations, however (having been made on Eadiolaria of very various 

 groups and supplemented by comparative anatomical considerations), allow us to draw a 

 general picture of the essential developmental processes in this great class. It may 

 probably be assumed that in all Eadiolaria, after maturation, the central capsule 

 discharges the function of a sporangium, and its contents are broken up into numerous 

 flagellate swarm-spores (zoospores). After these flagellate swarm-spores (resembling 

 Astasia) have emerged from the ruptured central capsule, they probably pass over into 

 a Heliozoan-stage (Actiriojihrys) and then after the formation of a jelly-veil into the 

 condition of SphcBrastrum. Afterwards, when a membrane is formed between the outer 

 jelly-veil and the inner nucleated ceU-body, an ^cima-stage arises, which exhibits in its 

 simplest form the differentiation of the spherical unicellular body into the central capsule 

 and calymma. Actissa thus represents both ontogenetically and phylogenetically the 

 primitive condition of the Eadiolarian organism, and may thus be regarded as the point 

 of departure of all other forms. 



142. The Astasia-Stage. — The formation of flagellate zoospores in the mature central 

 capsule is probably to be regarded as the common form of individual development in all 

 Eadiolaria ; since the whole contents are utilised in the formation of these swarm-spores, 

 and since the extracapsulum takes no share in the process and perishes after they are 

 evacuated, the central capsule may be regarded as a sporangium (see note A, below). 

 The zoospores of the Eadiolaria generally arise in the following way : — the nucleus of the 

 unicellular organism, sometimes earl)^ sometimes late (and in several difierent ways, 

 §§ 63-70) breaks up into numerous small nuclei, and each of these surrounds itself with 

 a small portion of the endoplasm. Very often, perhaps generally, this endoplasm contains 

 one or several fat-granules and sometimes also a small oblong crystal ; from the protoplasm 



