GENERAL ZOOLOGY 



Oocyte 

 nucleus 



Sperm head 



1st meiotic 

 spindle 



Fertilization 

 membrane 



1st polar 

 body 



Peri vitelline 

 space 



B 



2nd meiotic 

 spindle 



2nd polar 

 body 



Female 

 pronucleus 



Male 

 pronucleus 



D 



E 



Fig. 5.9. Meiosis during oogenesis of a roundworm, Parascans eqiiorum (Ascans megalocephala 

 btvalens). A, a primary oocyte into which a sperm carrying the haploid number of chromo- 

 somes (two) has just penetrated. B, showing two tetrads, each containing four half-chromosomes 

 (chromatids), on the equatorial plate of the spindle of the first meiotic division. C, showing 

 the anaphase of the first meiotic division with two chromatids from each tetrad passing to each 

 end of the spindle. D, showing the two pairs of chromatids on the spindle of the second meiotic 

 division. E^ showing the nucleus of the mature female germ cell (female pronucleus), the first 

 and second polar bodies, and the nucleus of the sperm head (male pronucleus), which has been in- 

 active since its entrance. The meiotic spindles in Parascans do not have centrioles or astral 

 rays. A perivitelline space is formed between the oocyte and the fertilization membrane which 

 arises after the sperm enters. All figures x690. 



zoon is specialized for motility and contains one-half as many chromosomes as 

 the primordial germ cells and somatic cells contain, one chromosome corre- 

 sponding to each of their pairs. Four functional spermatozoa are derived 

 from each primary spermatocyte. 



The process of oogenesis, or maturation of the female germ cell, is entirely 

 comparable to that of spermatogenesis insofar as nuclear changes are con- 

 cerned; the cytosomal specialization differs (Figs. 5.7 and 5.9). The un- 

 diflferentiated germ cells found in the ovaries are known as oogonia and give 

 rise to other oogonia during the period of division. Each of these presently 

 enters its period of growth, and the homologous chromosomes pair in synapsis 

 and become duplicated to form tetrads. This is the time when cytosomal 



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