THE ONTOGENESIS OF VERTEBRATES 57 



a single mass, and is spoken of as the " first polar globule " 

 in distinction from that resulting from the second division, 

 which is termed the " second." Strictly speaking, the first 

 and second polar globules are not equivalent, but the first is 

 the equivalent of two abortive eggs and the second of but 

 one ; and corresponding to this the first polar globule pos- 

 sesses twice the number of chromosomes exhibited by 

 either the second globule or the functional egg. Furthermore, 

 the two polar globules are frequently not extruded until after 

 the entrance of the spermatozoon, the presence of which seems 

 to act as a stimulus for these cell divisions. In these cases, 

 the unfertilized " egg " is, strictly speaking, not the ovum, 

 but the oogonium, which requires the two reductive divisions 

 to become the equivalent of the spermatozoon. 



To illustrate this by an actual example, let us suppose an 

 animal that possesses in each somatic cell sixteen chromosomes. 

 The spermatogonium t would thus possess thirty-two which, by 

 the reductive divisions, would result in the formation of four 

 spermatozoa, each with eight. Similarly the oogonium would 

 possess thirty-two chromosomes, a number which would be 

 reduced to sixteen by the expulsion of the first polar globule, 

 the latter body having the like number. The second reductive 

 division would result in the formation of a second polar body 

 with eight chromosomes, and would leave eight in the egg. 

 This number, when added to the same amount introduced by 

 the spermatozoon, restores the normal number, sixteen, and 

 thus forms the first cell of the new organism, equipped with 

 the regular somatic number, one-half from either parent. 



In this is seen a provision for avoiding that enormous in- 

 crease in the number of chromosomes that otherwise must be 

 the inevitable result of each conjugation. Furthermore, when 

 taken in connection with the fundamental law of heredity that 

 in the long run the two parents are equally potent in trans- 

 mitting their characteristics to their offspring and that neither 

 sex has the preponderance of influence in this direction, it is 

 seen that the hereditary substance must lie in the chromosomes 

 alone, since these are the only elements in which both parents 



