CtAMETOGBNESIS of ORNITHORHYNCHUS 485 



In subsequent development the cavity becomes more and 

 more tilled with connective tissue, and this, together with the 

 growth of the cortical walls of the ovary, caused the primitive 

 type of arrangement to be disguised and partly obliterated ; 

 but it should be pointed out that the lacunae figured on 

 PL 13, fig. 4, are largely the remains of the early cavity within 

 the gonad. 



8. The Size op the Largest and Smallest Ovarian 

 Oocytes of the Platypus. 



In the adult ovary of Ornithorhynchus no oogonia are to 

 be found ; all these seem to have undergone their maturation 

 prophases and to have become oocytes certainly long before the 

 animal is full grown. Even in one very small immature ovary 

 in Professor Hill's possession there were no oogonia ; this 

 ovary measured only 3 mm. in depth (see PL 12, fig. 2), whereas 

 the adult ovary is at least 12 mm. in depth. Possibly during 

 an embryonic period all the oogonial divisions, as well as the 

 prophases of the maturation division, have taken place, so that 

 when the animal hatches there are already formed all the 

 oocytes which it will possess and use during its life. 



This feature, with regard to the absence of true oogonia in 

 the ovary, does not occur in forms like the frog, where numerous 

 pockets of true oogonia exist in the ovary of the adult (vide 

 Gatenby, 9). Were it not for these pockets of cells which 

 continually proliferate new oocytes, the frog would be unable 

 to lay three to five thousand eggs for so many seasons. In 

 the case of Ornithorhynchus and other Mammalia, the number 

 of offspring produced is so small as not to necessitate a con- 

 tinuous new supply during each breeding season. 



Measurements have been taken of a number of the oocytes 

 of the smallest dimensions I could find. The smallest was 

 0-07 mm., the average among the smaller being 0-08 mm. In the 

 adult ovary the smallest oocytes measured from 0-08 to 0-09 mm. 



With regard to full-grown ovarian oocytes the largest I found 

 was 4-5 mm. in diameter, not counting the theca (PL 12, fig. 3); 

 4 mm. seems an average diameter for the ovarian oocyte of 



