28 J. P. HILL. 



interpret simply as coagulura in the perivitelline space, whilst 

 the so-called "segmentation nuclei" {n-^, n^) situated in it are 

 probably the polar bodies or their derivatives. The part 

 labelled t/o, and designated " white yolk/' I would regard as 

 the ovum itself. It exhibits an obvious differentiation into a 

 central vacuolated area and a peripheral, dense, granular 

 zone with scattered vacuoles, and I thiuk there can be little 

 doubt but that the former corresponds to the deutoplasmic 

 zone of the Dasyure ovum, the latter to the formative zone. 

 It is these errors of interpretation apparently which misled 

 Caldwell into making the statement, now widely quoted in 

 the text-books, that cleavage in Phascolarctus is of the 

 meroblastic type. 



Chapter III. — Cleavage akd Formation of the Blastocyst. 



1. Cleavage. 



Cleavage begins in the uterus as in Didelphys, Phasco- 

 larctus, and no doubt Marsupials in general. The first ex- 

 ternally visible step towards it consists, as already described, 

 in the elimination by abstriction of the deutoplasmic zone at 

 the upper pole. The yolk-body so formed appears as a 

 definitely liiuited, clear, rounded mass which lies in contact 

 with the slightly concave upper surface of the formative 

 remainder of the ovum. It is quite colourless and transparent 

 except for the frequent occurrence in it of a small, more or 

 less irregular opaque mass, representing- probably a condeusa- 

 tion product of its fluid material (cf. PI. , tigs. 8, 14, y.h.). 

 Consisting as it does of a very delicate cytoplasmic reticulum 

 with fluid-filled meshes it is extremely fragile, and is seen to 

 advantage only in fresh material (figs. 14 and 19, y.l.). It 

 takes no direct part in the later developmental processes* 

 though during the formation of the blastocyst it becomes 

 enclosed in the blastocyst cavity and finally undergoes dis- 

 integration therein, its substance becoming added to the fluid 

 which fills the same, so that it may be said, in this indirect 

 way, to fulfil, after all, its original nutritional destiny. Separa- 



