i 4 2 COMPARATIVE ANATOMY 



observed to encyst and divide into a number of minute young 

 forms which are subsequently set free by rupture of the cyst- 

 wall. Paramotba eilhardi, a form closely allied to Amoeba, 

 encysts and divides into a number of segments which emerge 

 from the cyst as minute bean-shaped, actively swimming 

 bodies, each provided with a pair of long vibratile proto- 

 plasmic filaments termed flagella. These zoospores, as they 

 are called, divide with nuclear mitosis, and eventually lose 

 their flagella and become amoeboid, but they have not been 

 observed to conjugate. 



The large and peculiar multi-nuclear Rhizopod, Pelomyxa 

 lacustris has an interesting life-history, culminating in the 

 formation of a number of pseudopodiospores, which for a 

 while put forth pseudopodia, ingest food and lead an inde- 

 pendent existence, but are incapable by themselves of growing 

 up into the parent form. To this end it is necessary that 

 two pseudopodiospores (also called gametes, from the Greek 

 yajueTT/s, a spouse) should conjugate. They come together 

 and fuse completely, nucleus with nucleus and cell-body with 

 cell-body. The resulting individual is known as a zygote 

 (fvywros, joined together), and develops into an adult 

 Pelomyxa. The reproductive processes in this organism are 

 heralded by peculiar nuclear changes, the details of which 

 are too complicated to be dealt with in this place, but it is an 

 interesting and significant fact that prior to the formation of 

 the pseudopodiospores or gametes, each of the many nuclei 

 contained in the parent Pelomyxa undergoes two mitotic 

 divisions, in which definite chromosomes make their appear- 

 ance. The first of these is a meiotic or reducing division, the 

 second an equal division, but with half the number of chromo- 

 somes that made their appearance in the first division.* 



It is obvious that the conjugation of the two gametes is 

 strictly comparable to the fertilisation of an ovum by a sper- 

 matozoon, and that the preparatory nuclear changes are similar 

 in each case. In other words, in an organism that is indis- 

 putably a Protozoon of simple structure, we find a fully differ- 

 entiated sexual process, essentially the same as that described 

 for the frog, and differing from the latter chiefly in the fact 

 that the two gametes are equal in size and indistinguishable 



* K. Bott, Ueber die Fortpflanzung von Pelomyxa lacustris. Archiv 

 fur Protistenkunde viii. 1907 



