126 



MANUAL OF ELEMENTARY ZOOLOGY 



B 



are known as trophozoites. When they are full grown, 



two of them come together and form them- 



Reprocluotlon. sdveg into & rounde d ma ss without fusing. 



Around this mass a double cyst is secreted. Each 

 individual now divides by multiple fission, in which the 

 mitosis resembles that of the frog 

 in that the centrosome appears 

 outside the nucleus and the nuclear 

 membrane disappears. There 

 arise thus, as in the spore forma- 

 tion of Amceba, a number of small 

 germs, a certain amount of residual 

 protoplasm being left, which is 

 absorbed by the germs during 

 their development. The germs 

 conjugate in pairs, in which one 

 member is derived from each 

 parent. Thus, although the par- 

 ents are to all appearance exactly 

 alike, there happens here what 

 happens also in the frog, where 

 the parents are unlike, namely, 

 that the gametes are derived from 

 distinct parents. This is known 

 as cross-fertilisation, and is found 

 in the vast majority of cases 

 throughout the animal kingdom, 

 though instances do occur of what 

 is known as self-fertilisation, in 

 which gametes derived from the 

 same parent unite. 



It is said that in M. magna the 

 germs from the two parents are 

 alike, but in M. agilis those of 

 one parent — the "female" — are 

 rounded, and those of the other — 

 the "male" — pear-shaped. Each 

 zygote is known as a sporont ; it now 



Fig. 76. — Monocystis. 



^ZT^A^t 2S secretes a boat-shaped, horny case, 

 of spermatozoa, the offspring of and is known as a pseudonavicella. 



the sperm mother cell in which w-ii,- t i,„ •. j. -j , 



it was embedded. Within the case it divides by 



