SEX DETERMINATION IN DINOPHILUS GYROCILIATUS. 329 
The Problem of Sex Determination in 
Dinophilus gyrociliatus. 
Part 1. — The Sexual Cycle. 
By 
Cresswell Shearer, UI.A , 
Clare College, Cambridge. 
With Plates 30-34 and 5 Text-figs. 
Contents. 
PAGE 
1. Introduction ...... 329 
2. Material, Habits and Methods .... 336 
3. Fertilisation and Early Oogenesis .... 348 
4. Late Oogenesis and Formation of the Male and Female Eggs . 357 
5. Maturation Stages . . . . 359 
(a) In the Male Egg .... 361 
(b) In the Female Egg . . . . 362 
6. Conclusions and Summary . 362 
1. Introduction. 
The problem of sex-determination in Dinophilus g} r ro- 
ciliatus is concerned, first, with the investigation of the 
factors involved in sex-determination in the sexual or fertilised 
egg ; and secondly, with those involved in sex-determination 
of the parthenogenetic egg. In these two conditions the 
factors seem different, and for this reason are perhaps better 
considered separately. In the present paper only those con- 
cerned in sex-determination in the sexual egg are dealt with ; 
while those involved in the parthenogenetic cycle, if it really 
exists, are left for a future paper. 
