Embryology. 11 



into one cell. The union of these two cells is called the act of fertiliza- 

 tion or impregnation. The female egg cell, or ovum, in mammals, is 

 globular and is surrounded by a covering of transparent matter called 

 the zona pellucida, or vitelline membrane, on the outside of which is 

 deposited a coating of mucous albuminous matter. ( These two subse- 

 quently amalgamate into one membrane called the prochorion, and this 

 afterwards disappears and is replaced by the clwrion.} The interior 

 part of this cell is filled with 'protoplasm enveloping the nucleus or germ. 



FIG. 10. Primitive egg cell of 

 chalk sponge performing amoeboid 

 movements. Four consecutive posi- 

 tions. 



FIG 11. 



FIG. 11. B. -Primitive egg cell 

 of Hermit Crab, in 8 positions 

 assumed by itself. 



FIGS. 12 & 13. 



FIG. 12. C. Primitive egg cell of cat, in 5 positions. 



FIG. 13. D. Primitive egg of trout. E. Primitive egg of hen. F. Primitive egg of 

 man. 



All the ova or egg cells of mammals, including those of man, are 

 substantially alike in size, shape, structure, and chemical composition. 



The male cell or spermatozoon, as it is called, is much smaller than 

 the female cell. But it is of the same force value and the same chem- 

 ical equivalence. It is somewhat oval or elongated, and from one end 

 there projects a filament or tail of protoplasm. This tail moves, or vi- 

 brates, with the restlessness and force characteristic of and innate in 

 protoplasm, and in consequence of its peculiar shape this vibration 

 causes a progressive movement to the cell exactly like swimming. It is 

 this circumstance that caused the first observers to think it was a com- 

 plete animal in itself and to give it the name spermatozoon (or seed 

 animal). 



When the spermatozoon and ovum are brought into contact with each 

 other, the former, under the impulse of a powerful attraction, penetrates 

 the cell walls of the latter, the two protoplasms are thoroughly mingled 

 and the two nuclei coalesce and re-form into a new nuclus or germ spot. 



