46 INVERTEBRATE MOBPSOLOOT. 



the oviduct, leading to the exterior. At the tip of each tube 

 the primitive germ-cells (Fig. 20, g) are located, and lower 

 down ova (o) in various stages of development towards matu- 

 rity are to be found, each surrounded by a number of small 

 undeveloped germ-cells, known as follicle-cells (/), whose func- 

 tion it is to transfer food-yolk (y) to the growing ovum. As 

 the latter approaches maturity the follicle-cells secrete around 

 it a thick, sometimes highly sculptured shell and finally 

 degenerate. 



As a rule, conjugation with a spermatozoon, i.e. fertiliza- 

 tion, is necessary as an antecedent to further development. 

 Before this takes place, however, certain modifications of the 

 ovum are necessary, the phenomena which accompany them 

 being known as the maturation of the ovum. In this process 



A B 



■ Fia. 21.— Diagrams illustrating the Maturation op the Ovum. 



A = formation of the first polar globule (pg). 



B = formation of tiie second polar globule and entrance of the sperm-nucleus 



(Fig. 21, A) the nucleus approaches the surface of the ovum 

 and there undergoes a karyokinetic division which is peculiar 

 in that in the equatorial-plate stage twice as many chromo- 

 somes are formed as are typical for the species. These do not 

 undergo longitudinal division, and by the karyokinesis their 

 number is reduced to the typical number, a small cell, the 

 polar globule (pg), being separated from the ovum with half 

 the chromosomes, while the others are retained within the 

 ovum. The nucleus of the ovum, instead of now returning 

 to the resting stage, divides again (Fig. 21, B), a second polar 

 globule being formed and receiving half the chromosomes 



