DEVELOPMENT OF THE OPOSSUM Da) 
and six normal vesicles with thin, partly bilaminar walls. Of 
his unsegmented ova I think all were unfertilized: Hence the 
42-celled and the 68-celled blastocysts which Selenka describes 
are the youngest of his specimens which approach a normal 
opossum egg. These two are practically normal except for the 
shrinkage of the vesicle from the vitelline membrane and for the 
regular gradation in size of the blastomeres from one pole to 
the other—a condition entirely accidental and not at all char- 
acteristic for this or any other stage in the development of the 
opossum. His pear-shaped, thick-walled vesicle with spreading 
entoderm (Selenka, ’87, Fig. 1 and 2, Taf. XVIII) is clearly a 
degenerating specimen, as I judge by comparison with num- 
bers of similar preparations from my collection. Whenever, 
in any batch of eggs, there are very retarded specimens, these 
are to be regarded with suspicion. Many such abnormal eggs 
can be seen in my photographs of living eggs, as, for example 
in figure 4, plate 1, and figures 3 and 4, plate 11. Selenka’s in- 
terpretation of certain gaps in the walls of his young blastocysts 
as the ‘blastopore’ must be rejected for the reason that these gaps ~ 
are not to be found in completed blastocysts, of which I have a 
hundred specimens. Where openings in the blastocyst wall do 
exist In young specimens, they are easily explained when the 
method of blastocyst formation is understood. 
On the origin of the entoderm in the opossum Selenka is not 
clear. I must support one of his suggestions, however, for his 
designation ‘Urentodermzelle’ as applied to the large cell in his 
42- and 68-celled eggs expresses its true function. I previously 
described the rather constant occurrence of such cells, all in an 
excellent state of preservation; but in the absence of the suc- 
ceeding transitional stages, I rejected the view that these are 
true entoderm mother cells and considered them of ‘nomor- 
phological importance.’ I am now enabled to give a complete 
account of the most interesting behavior and the destiny of these 
cells. 
On the time relations in the development of the opossum my 
data substantiates Selenka’s account only in regard to the time 
between copulation and parturition, which is thirteen days. 
