6 CARL GOTTFRIED HARTMAN 



the early blastocysts also represent normal stages of the opossum 

 embryos. With so little material before him, some of which 

 was indeed not entirely normal, it is no wonder that Selenka 

 should have wrongly interpreted in every essential detail the 

 cleavage of the opossum egg. 



Mention should be made in passing of a short paper by Minot 

 ('11) in which the late bilaminar blastocyst of the opossum is 

 described in some detail. The article, however, adds little, if 

 anything, to the work of Selenka, which it entirely ignores. 

 Indeed, the author fails to recognize the shell-membrane, as did 

 Selenka, and in his interpretation of Hill's ('10) work falls into 

 other minor errors. The article by Dr. Minot will be discussed 

 in Part III of these studies, to be pubhshed in a subsequent 

 number of this Journal. 



2) Cleavage and blastocyst formation in Dasyurus as described 

 by Hill '10). Inasmuch as Professor Hill's thorough study of 

 the Australian 'native cat' furnished a strong impulse for the 

 study of the development of the opossum and since compari- 

 sons will be made in this paper between the two forms, it is deemed 

 desirable to review in some detail the developmental processes 

 as described by the British author. 



The ovarian ovum of Dasyurus, aside from its much greater 

 size and larger amount of yolk as compared with Eutherian 

 ova, exhibits nothing unusual before the period of maturation. 

 During the period of growth the ovum is more or less homogene- 

 ous throughout, or perhaps somewhat centrolecithal. But at 

 the time of formation of the first polar body, which is given off 

 in the ovary as in the Eutheria, the egg takes on a striking po- 

 larity through the accumulation of surplus yolk at the animal 

 pole of the egg. The egg maintains this condition during its 

 passage through the oviduct, in which it adds the albumen layer 

 and the shell and where insemination takes place. The pronuclei 

 and the first cleavage spindle lie in the granular formative cyto- 

 plasm, which occupies about two-thirds of the egg and which 

 appears in longitudinal section as a broad crescent, the two 

 horns of which partly envelop the yolk mass (text fig. 4 A, see 

 page 34). 



