THE Ex\RLY DEVELOPMENT OF THE MARSUPIALIA. 75 



specific differences in the early development of these Marsu- 

 pials, especially in regard to the time of appearance of the 

 entoderm. In Dasyurus, it will be remembered, the primitive 

 entoderm cells first become definitely recognisable as inter- 

 nally situated cells iu vesicles 4*5 mm. in diameter. In 

 Perameles they occur in vesicles just over 1 mm. in diameter, 

 while in Macropus they are already present iu a blastocyst 

 only '35 mm. iu diameter, so that it would appear that the 

 entoderm is diifereutiated much earlier in the higher, moi-e 

 specialised types than in the more generalised forms. This 

 difference in time of appearance of the entoderm is perhaps 

 to be correlated with a difference in size of the ovarian ova 

 in the three gfenera mentioned. 



1. Perameles. 



The earliest material of Perameles I possess consists of two 

 eggs of P. obesula, which I owe to the skill and enthusiasm 

 of my friend Mr. S. J. M. Moreau, of Sydney. Egg A 

 measures '23 mm. in diameter, and egg B, '24 x *23 mm. 

 The former consists of thirty-two cells, the latter of thirty. In 

 both the shell-membrane has partially collapsed, but the general 

 plan of arrangement of the blastomeres can still fairly readily 

 be made out. Fig. 51, PI. 3, represents a micro-photograph 

 of a section of egg b, the better of the two. It shows the 

 shell-membrane (nearly '005 mm. thick) externally, con- 

 siderable remains of the albumen between that and the 

 deeply stained zona, and then, closely applied to the inner 

 surface of the latter, the blastomeres arranged in the form of 

 an inverted D, so as to enclose a central space, open below 

 as the figure stands. This latter opening extends through 

 the series, and it seems probable tliat there was a corres- 

 ponding one opposite to it in the intact egg. Evidently we 

 have here a stage in the formation of the blastocyst, in which 

 the blastomeres are in course of spreading towards one or 

 both of the poles of the sphere formed by the egg-envelope^. 



