12 University of California Publications in Zoology [Vol. 18 



has been completed (pi. 1, %. 2), but these cytoplasmic changes have 

 come about long before the division of the centrosome (pi. 1, figs. 5, 6, 

 7). The division of the blepharoplast in trichomonad flagellates ini- 

 tiates mitosis, and the question now arises as to the significance of this 

 division in Giardia microti at the time when mitosis has begun, yet 

 previous to the division of the centrosomes. 



If we consider that Giardia has evolved as a two-celled individual 

 from a unicellular trichomonad flagellate which had undergone mitosis 

 but no plasmotomy or division of the cytoplasmic body, then this point 

 can be easily explained. In the mitosis of trichomonad flagellates, as 

 has been said before, the blepharoplasts initiate mitosis by dividing and 

 forming a paradesmose between the two daughter blepharoplasts ; these 

 blepharoplasts become the poles of the spindle. They appear in func- 

 tion to be centrosomes as well. However, they still act as the central 

 point at which the new undulating membrane and the new flagella 

 will form and so must be considered blepharoplasts (see Kofoid and 

 Swezy, 1915). It is very significant that in the anaphase these 

 daughter blepharoplasts temporarily divide to form two smaller gran- 

 ules, one a ' ' centrosome ' ' and the other a ' ' basal ' ' granule or blepharo- 

 plast with the paradesmose connecting the basal granules or blepharo- 

 plasts. This paradesmose in all probability corresponds in part to the 

 anterior commissure in Giardia microti, the fibril connecting the two 

 blepharoplasts (pi. 1, fig. 1), which has here become a permanent 

 structure. It is thus evident that Giardia is a two-celled animal 

 derived from a trichomonad flagellate which had gone through mitosis 

 but not plasmotomy. In the trichomonad flagellates these new centro- 

 somes are only temporary structures and in the telophase the separated 

 centrosome and blepharoplast reunite to become the permanent ble- 

 pharoplast. Hence this blepharoplast contains the centrosome. In 

 Giardia this fusion of the centrosome and blepharoplast has not taken 

 place, but the separation is permanent, each centrosome occupying a 

 position on the nuclear membrane at the anterior pole of each nucleus 

 while the blepharoplast becomes a separate organ at the head of the 

 axostyle (pi. 1, fig. 1). 



Now then, when the axostyle does divide at the time of mitosis in 

 the prophase in Giardia, this division represents only the delayed 

 division of this part which would ordinarily have taken place in the 

 trichomonad stage of the organism after the completion of mitosis. 

 This structure is now divided to provide the two axostyles of the new 

 daughter individuals which will later separate after mitosis by plas- 



