E. MARSHALL!. THESOCYTES. 175 



îng tliemselves and becoming dispersed from the congregated 

 state. The groups are evidently not the result of the coming 

 together of the thesocytes from neighboring parts, for I have been 

 able to trace back their origin to certain archœocyte congeries 

 described in the last chapter. The arch?eocjtes develop into the 

 thesocytes by gradually accumulating the fat-like bodies in the 

 protoplasm, whereby the cells grow considerably in size. PL IV, 

 fig. 24 illustrates the transition of a mass of archœocytes into 

 that of thesocytes. On the left of that figure are seen a number 

 of archceocytes still in the original state ; toward the right they pass 

 into thesocytes by an uninterrupted series of intermediate stages. 



As before mentioned, the orange-yellow spots, i.e., the 

 thesocyte-masses, are very variable in size. Some are quite 

 minute, while others may be as large as half a millimeter across. 

 This indicates that the transformation into thesocytes may take 

 place at any stage in the growth of archœocyte congeries. It is 

 probable that even a solitary arcliœocyte may develop into a 

 thesocyte ; but of this I have no direct evidence. The thesocyte- 

 masses were not noticed by me in any other species than in 

 E. marshalli, and it may be necessary in the former cases to 

 assume that the thesocytes there present in scattered distribution 

 arise from similarly circumstanced archœocyets. 



The question, whether all the archseocyte congeries in E. 

 marshaUi are destined to undergo the above development, is, I 

 believe, to be answered decidedly in the negative. (See ante, 

 p. 172, & anon under Reproductive Elements). While the 

 process is to be called perfectly normal, it appears to represent 

 a sioecial case of the functional and morphological differentiation 

 of archœocytes, taking place under certain physiological condi- 

 tions, which are difficult to determine but which at all events 



