THE EARLY DEVELOPMENT OF THE MAESUPIALIA. 



27 



difference in sectional plane. The figure is stated to represent 

 *'the seventeenth section of a vertical longitudinal series of 

 tliirty-five sections tlirougli the segmenting ovum, containing 

 two nuclei, taken from the uterus and measuring "17 mm. in 

 diameter." Caldwell has, I think, fallen into several errors 

 in his interpretation of the structural features seen in this 



-yfe 



o2b. 



Section of uterine ovum of Phascolarctus ciuereus. 

 (After Caldwell.) 



figure. In the first place, the sectional plane appears to me 

 not to be vertical as in my own figs. 11 and 12, but horizontal, 

 and to have passed through the lower portion of the deuto- 

 plasmic zone, shown in the figure as a central markedly 

 vacuolated area. Then there is no evidence to be derived 

 from the figure in support of the description of the ovum as 

 segmenting. The part inside the zona {cm.) labelled y^ and 

 described as "protoplasm with finest yolk-granules," 1 would 



