INTRODUCTION 



have been descriptions of supposed independent organisms sug- 

 gesting such intermediate character (Trichoplax and others), but 

 the true nature and history of these structures have not been placed 

 on a definite basis, and do not really admit of discussion. The 

 nearest case of a transitional form appears to be the Choano- 

 flagellate " Proterospongia " of Savile Kent, which has been 

 observed on several different occasions from different localities. 

 It combines in one colony " amoebocytes " and " choanocytes," but 

 it appears that the one form of cell develops into the other. It is 

 certainly not unreasonable to regard Proterospongia as a step 

 forward from the Choanoflagellata in the direction of the Parazoa. 

 There is no instance of equally definite character tending to 

 connect Protozoa of any class with the Enterozoa. 



Until recently it was possible to add to this distinction between 

 Protozoa and Metazoa the very striking one that all Metazoa 

 reproduce by means of fertilised egg-cells (as well as by other 

 processes), such fertilised cells being the result of the union of 

 specially developed egg-cells and sperm-cells. Conjugation of two 

 cells similar to one another as a preparation to multiplication by 

 fission was known and described in several Protozoa, but the special 

 units, the static female egg-cell and the motile male " spermatozoid," 

 were unknown in Protozoa. The apparent exception to this pre- 

 sented by some of the Volvocinean Flagellata was regarded as a 

 reason for assigning these organisms to the pedigree or great series 

 of Plants, thus removing them from association with the other 

 Flagellata. In the Plant series, though many groups both among 

 the highest and lowest do not present sexual reproductive elements 

 under the typical forms of egg-cell and spermatozoid (antherozoid), 

 yet some of the lowest and simplest, as well as some of the higher, 

 plants do develop motile conjugating " male " cells, which seem to 

 render the relegation of Volwx to the vegetable series a reasonable 

 proceeding. Within the last decade, however, we have not only 

 become acquainted among undoubted Protozoa with instances 

 of the development of " microgametes " or small conjugating cells, 

 which are distinguished by their size from the larger egg-cells 

 or " macrogametes " with which they fuse in order to form a 

 fertilised "germ," but we now know undoubted Protozoa which 

 exhibit the breaking up of a parent male unicellular individual into 

 a number of motile microgametes. These have the appearance and 

 characteristics of the spermatozoa of higher animals, are developed 

 from the parent male cell by the same steps as are spermatozoa 

 from sperm-mother-cells, and proceed to fertilise the female macro- 

 gametes in the same manner as occurs in the fertilisation of the 

 egg-cell in Metazoa. 



The Coccidiidae among the Sporozoa and certain of the Haemo- 

 flagellata are the Protozoa in which this phenomenon has been 



