FROM SIMPLE CELL TO COMPLEX ANIMAL 31 



around the eggs of birds and reptiles. Ova differ very greatly 

 in size. The largest are found among the birds. The ' ' yellow ' ' 

 of these eggs represents the real size of the ovum. Variations 

 in size are due not so much to a difference in the amount of 

 protoplasm as to a varying amount of food or yolk in the 

 cell. The food may be uniformly distributed throughout 

 the ovum, mingled with the protoplasm, or it may collect at 

 one pole, forcing the active protoplasm to occupy the other 

 pole (Fig. 9, C). The yolk furnishes food to the young indi- 

 vidual or embryo in its early development, that is, during cell- 

 division, before it can get food for itself. 



48. The spermatozoon or male element is ordinarily in 

 striking contrast to the female. It is typically very small* 



-n. 



PIG. 10. Types of spermatozoa. A, from the round worm (Ascaris) with a cap, somewhat 

 amceboid; B, from the Crayfish, with numerous projections; C, from Frog; D, from Sea-urchin.. 

 h, head; m, middle piece; n, nucleus; t, tail or flagellum. The head is chiefly nucleus. . 



Questions on the figure. What are the chief points of similarity and dissimi* 

 larity in these spermatozoa? How do they agree with, and how differ from, the 

 ova in Fig. 9? How do they differ from the average cell? What parts of the: 

 structure of typical cells are believed to be represented in the sperm cells? 



active, and with thin protoplasmic projections (Fig. 10). Struc- 

 turally, the typical spermatozoon consists of a "head" piece,, 

 a middle piece, and a ' ' tail ' ' or flagellum. The head is composed 

 chiefly of the chromatic material of the nucleus. A delicate- 

 covering of cytoplasm envelops the head and is drawn out into* 



