CHAPTER VI. 



GENERAL EMBRYOLOGY. 



THE vertebrate animal is developed from a fertilised ovum. 

 The multitude of cells of which together with cell products 

 the tissues of the adult organism are composed, is produced by 

 the repeated division and sub-division of the original oosperm. 

 Thus we may trace back the organism to the ovum with which 

 a spermatozoon has entered into fertile union ; and the develop- 

 ment of the ovum and spermatozoon in the parents we may 

 trace back to the fertilised ovum from which they also were 

 respectively developed. Omnis cellula a celluld is a law that, so 

 far as we know, holds good for all animal life. It may be that- 

 in past ages, it may be that even now' in the depths of the ocean, 

 or in stagnant pools, the origin of a living cell from not-living 

 matter has occurred or is taking place. No one has a right to 

 assert that it has not and cannot. But as a matter of science 

 such abiogenesis (or origin of living from not-living matter) has 

 never yet been proved. And therefore, as students of science, 

 we shall do well to accept the law of biogenesis omnis cellula a 

 celluld as our working hypothesis. 



The ovary and testis which in the parents produce the 

 ovum and spermatozoon arise in the first instance, when the 

 parent is still an embryo, as thickenings of the sheet of cells 

 that lines the body-cavity. The thickened part of the sheet is 

 called the germinal epithelium ; and for a while it is quite similar 

 in male and female. By mutual ingrowth of co 1 ls, (1) from the 

 germinal epithelium into the substance of the underlying body- 

 walls, and (2) from the body-walls into the germinal epithelium, 

 a thickened ridge, the genital ridge, is produced. But still there 



