Different Types of Sex Determination 465 



a system which often works just as well in dioecism as in Amphibia 

 and Melandriu7n-\ike types. 



One word should be added about the most aberrant case known 

 in animals, the classic case of Dinophilus (Korschelt), which Hart- 

 mann also terms "phenogenetic sex determination." The female of 

 the primitive worm Dinophilus apatris lays egg capsules containing 

 large and small eggs. The large ones develop into females; the small 

 ones, into males. Thus fertilization (i.e., the nature of the sperm) has 

 nothing to do with sex determination, which is already fixed in the 

 eggs before meiosis. The two types of eggs develop somewhat differ- 

 ently. Both are formed by union of a number of oocytes with resorp- 

 tion of all but one nucleus (the details are a little different for male 

 and female eggs ) . The future female eggs add a growth period, which 

 the future male eggs do not have. If females are prevented from being 

 fertilized (which is possible in some species by using a trick invented 

 by Beauchamp, 1910), the two types of eggs develop parthenogenet- 

 ically and again into females from large eggs and males from small 

 eggs. 



Though there can be hardly any doubt that here the two pure 

 sexes are not determined by a chromosomal mechanism, it does not 

 help much to call this "phenotypic sex determination." It cannot be 

 excluded that the female is XXY with a strong female determiner in 

 the Y, and that in oogenesis the Y is removed in future male eggs, 

 since the orderly removal of definite chromosomes ( heterochromatic 

 in Sciara, sex chromosomes in aphids, etc.) is a known phenomenon. 

 It could also be possible that in an XY female the small eggs have a. 

 directed first maturation division, always removing the Y, just as 

 Seiler (1920) was able to do in Talaeporia by temperature action; 

 while the unknown different chemical condition of the large eggs is re- 

 sponsible for always pushing the X into the first polar body. Also in 

 aphids we know of definite cytoplasmic influences upon the maturation 

 division in the sexuparous females (directed meiosis in aphids not yet 

 found). Parthenogenesis may or may not affect such a mechanism, 

 depending upon the type of parthenogenesis (e.g., diploid, haploid, 

 with and without autogamy, etc.). Before accepting the pseudo- 

 explanation of phenotypic sex determination, I am willing to wait for 

 a detailed cytological study. 



The best case known to me which could be claimed for phenotypic 

 sex determination is Buchner's (1954) work on the symbionts of a 

 coccid. Here the amazing fact was found that embryos which are 

 invaded by the typical fungoid symbionts of the species develop into 



