2 20 BIOLOGICAL LECTURES. 



come from the substance of the pole plates, and hence from the 

 nucleus, although his observations were somewhat incomplete 

 and his account obscure. A chromatin origin of a centrosome- 

 like body was also described by Balbiani in the nucleus of 

 SpirocJioiia gemmipara, while a nuclear origin of the centro- 

 some among the Metazoa has been described by Brauer for 

 the germ cells of Ascaris DiegaloccpJiala jinivalens, and by 

 Riickert for the germinal vesicle of Copepoda. 



In the last two cases, however, it seems obvious that such 

 an origin among higher metazoan cells can have little value in 

 problems relating to the origin of centrosomes, for here dif- 

 ferentiation is fully as complete as in any other cell, and the 

 origin of such centrosomes can give no light on the problem. 

 To a certain extent, Hertwig's view also is open to the same 

 criticism, although to a far less degree. It must not be over- 

 looked that ActiiiosphcBriuin is a highly differentiated proto- 

 zoon ; its nuclei are much more like the nuclei of Metazoa 

 than like most other Protozoa, and its reproductive processes 

 are far advanced along the line of sexual development. It 

 appears quite probable, therefore, that Hertwig's theory, al- 

 though exceedingly ingenious and important in the light of 

 certain facts to be mentioned later, does not go to the bottom ; 

 he picks up the thread at a point some distance from the spool, 

 but from that point he gives a satisfactory explanation, at least 

 from a tentative point of view, of many highly specialized 

 processes. 



Schaudinn's other alternative has opened the way for another 

 view which explains more primitive conditions in more primi- 

 tive organisms. This view, which I advanced in a previous 

 publication, is based upon the lower flagellates and rhizopods. 



It is not improbable that the condition of distributed nucleus, 

 as in Tetramitiis, is widely spread among the lower forms of 

 life. Biitschli ('96) described it for many of the lower algae 

 and for Bacteria, where the nucleus corresponds closely to that 

 of ordinary yeast cells, Saccharomyces cerevisece} In Tetrami- 



1 In these forms the chromatin granules lie scattered throughout the cell, or 

 aggregated in various ways in a cytoplasmic vacuole. Like Tetramitiis, they also 

 have a peculiar structure (the " nucleus " of Wager) in the cytoplasm, which may 



