THl'J EAi;r,Y DEVELOPMENT OF THE ilAESUPlALIA. 25 



trapods, unless we are to suppose that the Metatheria are 

 even now on the way to acquire secondarily the oviparous 

 habit, tnuch in the same way as the Monotremes^ according 

 to Hubrecht, have long since succeeded in doing. 



The occurrence of a shell-membrane round the Marsupial 

 ovum has also an important ontogenetic significance in rela- 

 tion to the mode of formation of the blastocyst, as I shall 

 endeavour presently to show. 



4. The Uterine Ovum. 



The unsegmented ovum from the uterus (figs. 8-13) 

 consists of the following parts : 



(1) The shell-membrane externally, •0016-"002 mm. in 

 thickness. 



(2) The laminated layer of albumen, •015-'022 mm. or 

 more in thickness. 



(3) The zona, about "0016 mm. in thickness. 



(4)' The perivitelline space, between the zona and the 

 ovum, occupied by a clear fluid which coagulates under the 

 action of certain fixatives, e. g. Hermann's fluid (fig. ll,2)..y.), 

 and which has diffused in from the uterus. The minute 

 polar bodies lie in this space, usually nearer the upper pole 

 than the lower. 



(5) The ovum proper. 



The entire egg is spherical in form, and varies in diameter 

 in the fresh state from about '3 mm. to '36 mm. (average 

 about '32 mm.). 



The ovum itself is ovoidal, its polar diameter always slightly 

 exceeding the equatorial. Its average diametrical measure- 

 ments in the fresh state run about "25 x '24 mm., though I 

 have records of ova measuring as much as '3 x '29 mm,, and 

 I find that there is an undoubted slight variation in the size 

 of the ova of even one and the same batch, as well as in those 

 from different females. 



The uterine ovum exhibits the same marked polarity as 



