THE EAiaY DEVELOPMEXT OF THE MAESUPIALIA. 11 



exteuded beyond the limits of the embryonal area. The 

 position of the " blastopore " is said to be marked in all by a 

 mass of coagulum attached to the wall, and in three by a 

 definite opening as well. It is situated excentrically in the 

 embryonal area. [Except for the "blastopore" and the 

 presence of a thick layer of albumen, this blastocyst stage is 

 quite comparable with the corresponding one in Dasyurus; 

 the latter, however, is considerably larger. Of Selenka's 

 early material, I think it is these blastocysts alone wiiich had 

 any chance of giving origin to normal embryos.] 



W. H. Caldwell, who, as Balfour student, visited Australia 

 in 1883-4, obtained a very rich collection of early marsupial 

 material, of which, unfortunate!}^, no adequate account has 

 ever been published. He gave, however, in his introductory 

 paper on the ' Embryology of the Monotremata and Marsu- 

 pialia' ('87), an account of the structure of the ovum, both 

 ovarian and uterine, in Phascolarctus, and he showed that 

 the ovum during its passage down the Fallopian tube becomes 

 enclosed outside the albumen layer in ''a thin transparent 

 membrane, "OOIS mm. thick," which he homologised with the 

 shell-membrane of the monotreme egg. This important dis- 

 covery of the existence of a shell-membrane in the Marsu- 

 pialia I can fully confirm. I am, however, unable to accept 

 his interpretation of the internal structure of the ovum 

 of Phascolarctus, or his remarkable statement that cleavage 

 in that form is of the meroblastic type. Cleavage is not 

 described in detail, nor is any account given of the mode 

 of origin of the germ-layers. 



Chapter 11. — The Ovum of Dasycrus. 



1. Structure of the Ovarian Ovum. 



The full-grown ovarian ovum of Dasyurus (PI. 1, fig. 1) 

 appears as a rounded, or more usually, ovalish cell, the 

 diameter of which varies in section in ten eggs measured 

 from -28 x -126 mm. to "27 x '26 mm. (average, "24 mm.), 

 and is therefore large relatively to the ova of Eutheria. It 



