11 



ary somatic mitosis. The chromosomes divide longi- 

 tudinally and the halves become the chromosomes of the 

 resulting grand-daughter nuclei. 



In some cases (e.g., Ascaris) each constituent of the 

 bivalent divides transversely into two, making the bi- 

 valents tetrapartite and thus forming what are known as 

 tetrads. The two bivalents which separate in heterotype 

 division, being thus divided across, are double and there- 

 fore dyads. In homotype each dyad becomes two monads 

 by dividing across at the point of transverse constriction, 

 which therefore corresponds to the longitudinal division 

 described in the preceding paragraph. 



The meiotic (heterotype) division separates the chromo- 

 somes which united in syndesis. 



In meiotic or reducing division the daughter nuclei get 

 dissimilar univalents, whereas in ordinary mitotic division 

 they get similar halves of univalents. This is the process in 

 spermatogenesis, and in oogenesis it is practically the 

 same. 



(Meiosis and Maturation are equivalent. See answer to 

 preceding question.) 



Define Fertilisation. Describe the Process. 



Penetration of the female gamete or ovum by the male 

 gamete or spermatozoon, followed by fusion of pront^clei, 

 and giving that " developmental stimulus " which causes 

 segmentation or cleavage to begin. 



The spermatozoon enters the ovum, sometimes at a 

 special opening, the micropyle, as e.g., in insect ova with 

 " shells." The pronucleus (nucleus reduced by meiotic 

 division) of the sperm fuses with the pronucleus of the 

 ovum to form the segmentation nucleus, and thus the full 

 number of chromosomes characteristic of the species is 

 restored. Segmentation or cleavage and the further de- 

 velopment of the ovum then proceeds. 



, In Protozoa the gametes may be only slightly different 

 or not different; and conjugation may only be temporary, 

 fertilisation being effected after exchange of pronuclei. 



