DEVELOPMENT OF THE OPOSSUM 9 



Briefly stated, the present paper, together with one soon to 

 follow, will show that the development of Didelphys differ from 

 that of Dasyurus, first in its indeterminate type of cleavage, and 

 second in the delay of structural evidences of polarity (aside 

 from the clue offered by the position of the polar bodies) up to 

 the time of entoderm formation. Agreement is found in the 

 presence of large quantities of yolk in the egg and in the manner 

 of entoderm formation. Other points of minor importance will 

 be discussed in the body of the paper. 



c. Material and technique 



1) Material. The specimen-s of opossum eggs and blastocysts 

 on which the present stud}^ (as well as two other articles shortly 

 to appear) is based, represent collections made during three 

 seasons, but especially during the winters of 1913-1914 and 1914- 

 1915. During the first season, 1912-1913, the collections made 

 resulted chiefly in establishing the breeding season of Didelphys 

 for this locality. About 140 females were used, some of which 

 were killed from time to time during the first season. The large 

 number of specimens required to yield the desired stages is due, 

 in part, to certain difficulties which were at first experienced in 

 breeding the animals in captivity. It is my intention to say 

 more concerning this matter in a subsequent publication on the 

 physiology of reproduction of the opossum. 



The present account deals with the unsegmented eggs, cleav- 

 age stages and blastocysts taken from twenty different animals. 

 This number includes Female No. 112 which furnished only 

 unfertilized and degenerating eggs ; but the specimen is here added 

 for a special purpose, as will appear below. 415 eggs were re- 

 moved from the uterus or the oviduct of these animals, making 

 an average of 22 eggs from each animal. The extremes in num- 

 ber were 1 and 45. No. 76 yielded 10 eggs from the oviduct on 

 one side only and No. 144 yielded 11 from a single uterus; No. 

 117 discharged 43 eggs from the right ovary exactly one month 

 after the left ovarv had been removed. 



