THE HIGHER ORGANISMS 119 



A beautiful example of this is shown in the case of 

 Volvox globator. This little plant begins its life history 

 as a hollow sphere composed of cells which at first show 

 no essential differences, are somewhat polyhedral in 

 form, and separated from one another by a hyaline 

 material. About the whole mass there is a distinct 

 membranous formation. Among the component cells 

 one larger than the others can early be distinguished. 

 As the total number of cells continues to increase and the 

 hyaline intercellular substance likewise to increase, this 

 particular cell enlarges and then divides by successive 

 segmentations until as many as sixty-four or even one 

 hundred and twenty-eight similar cells have been formed. 

 These are the reproductive cells. As their fertiliza- 

 tion is accomplished, they project into the central cavity 

 of the hollow organism, eventually divide into many 

 cells, of which a similar hollow sphere is formed which 

 remains in the body of the parent until upon its disso- 

 lution a number of young spheres are simultaneously 

 set free to repeat the cycle described. 



We have no right to regard anything that takes place 

 in nature as accidental simply because we see no purpose 

 in it or reason for it. We must not, therefore, hastily 

 conclude that loose combinations of organisms, such as 

 we see in Streptococcus, Spirogyra, Epistylus, Carches- 

 ium, Microgromia, etc., are unimportant to the organisms 

 concerned. 



By having many cells a division into somatic and 

 germinal forms is quickly followed by the development 

 of the former as the hosts or caretakers of the latter. 

 Such a primary differentiation is in turn soon followed 



(nu), pseudopods (psd). and flagella (fl). The endoderm cell to the right has 

 ingested a diatom (a), and all enclose minute black granules. C, two of the 

 large ectoderm cells, showing nucleus (nu) and muscle process (m. pr). D, an 

 endoderm cell of H. viridis, showing nucleus (nu), numerous chromatophorea 

 (chr), and an ingested nematocyst (ntc). E, one of the larger nematocysts 

 with extruded thread barbed at the base. F, one of the smaller nemotocysta. 

 G, a single sperm. (A, B, C, E after Parker, "Lessons in Elementary Biology": 

 D after Lankester; F and G after Howes.) 



