24 Cutler, Parthenogenesis in Animals 



the observations connecting a particular chromosome with the 

 determination of one sex are in mjany cases indisputable; there 

 is no evidence to show howj this chromosome acts; and that 

 since the sex of the offspring is in some cases modified by the 

 environment, it is probable that the presence of the chromosome 

 is associated with a particular kind of cell metabolism, of which 

 sex is to be regarded rather as a visible expression than a cause. 



Dinophilus.—\ have left to the end a brief account of the 

 mode of development of these animals, because it is so curious 

 and unparalleled by any other organism that it is impossible 

 at present to connect it with any scheme of sex determination. 



Two kinds of eggs are laid, differing in size; from the larger 

 one females develop and from the smaller one males. These two 

 types of eggs are laid in the) same capsulje.; It was formerly 

 thought that both these eggs required to be fertilised in order to 

 develop, but the recent work of Shearer shows that this is not 

 the case, for the larger female-producing eggs conjugate with the 

 sperm nucleus, while in the male-producing eggs no conjugation 

 occurs. 



In Dinophilus gyrociliatus, the species on which Shearer 

 worked, the rudimentary males hatch from the egg in a very 

 short time, and are sexually mature. The females, on the other 

 hand, when they hatch are in a larval condition, and definite ova 

 are at this stage not yet developed. In spite of this, copulation 

 at once occurs, and the spermatozoa find their way to the place 

 where ova will be formed. As soon as the primitive oogonial 

 cells appear they are at once joined by the sperm which 

 penetrates into the cells. The sperm nucleus becomes situ- 

 ated close to the oogonial nucleus, and . when the cells 

 divide both nuclear elements divide simultaneously. This 

 continues for from forty to fifty divisions. Occasionally, 

 however, the female nucleus divides first and the male nucleus 

 is excluded from one of the daughter cells which are produced. 

 Thus a condition is brought about in which some of the cells 

 -contain the whole of the male nucleus together with half the 

 female nucleus, while others contain only half the female nucleus. 

 This is the stage ajt which sex is determined : for those cells 

 containing male and female elements will become female-pro- 

 ducing eggs, while the male-producing eggs are developed 

 from, the oogonial cells containing only the female element. It 

 will be noted that in the cells containing both male and female 

 nuclear elements no fusion has at present taken place, and it is 

 not until a late development of the r egg that fusion of the male 

 and female nucleus occurs. 



Further, in neither the male or female eggs has there been 

 any trace of maturation divisions. These divisions do occur. 



