478 r. A, POTTS. 



sliiffciiio- of the gonad during growth from a median to a 

 posterior situation, and the conversion of a double rudiment 

 into a single mature organ. In tlie most advanced male 

 larva) the gonad is completely occupied by a brownish mass 

 of spermatozoa save for an apical cluster of indifferent cells 

 (inset to M'ext-fig. 11^ a), and zur Strassen supposes that 

 when the larvae begin to grow rapidly these cells proliferate 

 and foi'iu an ovary. In a single example ot "75 mm. length 

 (Text-lig. 11, b) the testis was represented by a receptaculum 

 seminis full of spermatozoa, and this was succeeded by an 

 ovary still slightly developed and oidy posteriorly situated. 

 ]n the jidiilt (Text-tig. 11, c) the growth in size ot" the 

 gonad has been so enurnioiis that the wlude of the body- 

 cavity is occupied by it. The ovary and oviduct together 

 form a narrow tube running twice the length of the body. 

 Then succeed the receptaculum seminis, and lastly, the uterus, 

 with a diameter nearly equal to that of the animal itself, runs 

 from near the anterior end to the genital aperture. The 

 great difference between this and the intermediate stage has 

 been effected bv the growth of the uterus with the fertilisa- 

 tion of the e"'0's. 



Though in the absence of other intermediate forms it is 

 impossible to produce clear proof that events take their course 

 as indicated above, yet it is probable that the female sex, 

 though represented by larva?, disappear without functioning, 

 while in the males, after the spermatozoa have been formed, 

 ova are produced in large quantities by the residual cells of 

 the gonad. The evidence for the derivation of hermaphro- 

 ditism in Rhabditis and Diplogaster from the female, and 

 in Bradynema from the male, is in both cases of essentially 

 the same nature, and depends on — 



(1) The recognition both in the original sex and the 

 hermaphrodite derived from it, of a constant pattern of 

 reproductive organ. 



(2) The discovery that the gonad of one sex is capable of 

 developing the gametes of the other sex. 



If zur Strassen's explanation is accepted, then in the limits 



