44 CARL G. HARTMAN 
egg in a single cleavage plane and that secondarily a shifting 
sets in early in the process of division. 
There is a third possibility, as described by Sobotta (’95) 
for the mouse. According to this author, if I follow him 
correctly, one of the two blastomeres of the 2-celled egg divides 
meridionally, but the other blastomere has the division spindle 
at right angles to the first cleavage plane. In other words, the 
cleavage plane of the first blastomere to divide stands at right 
angles to the first cleavage plane, whereas in the second blasto- 
mere it is parallel to it. In such a case some shifting is also 
necessary to bring about the typical crossed arrangement of the 
4-celled egg. This method does not obtain in the opossum, as is 
seen from my description above. 
This point would appear to be further complicated by Spur- 
geon and Brooks (’16), who describe and figure cleavage stages, 
derived apparently from two female opossums. According to 
these authors, the second cleavage plane passes through both 
blastomeres equatorially and not meridionally, and thus a 
fourth method is suggested. I would cheerfully accept the 
authors’ conclusions, but for the fact that the eggs described 
by them do not appear to me te represent normal fertilized 
eggs. I believe their specimens to be fragmenting and unfer- 
tilized eggs that have been in the uterus three or four days. 
My reasons are as follows: 1) Cases of fragmenting eggs are 
extremely common in cage animals and such eggs may fragment 
into regular pieces resembling blastomeres of eggs in cleavage, 
as I have seen repeatedly in hundreds of such eggs (compare my 
photograph in fig. 5, pl. 11). 2) In their illustrations some of 
the blastomeres have an additional peculiar nucleus and many of 
the nuclei are very eccentric in position. Multinucleated ‘cells’ 
and those with nuclei placed at a distance from their centers are 
quite characteristic of fragmenting eggs. 3) The ‘polar bodies’ 
represented are peculiar for their large area in cross-section and 
for their position at a distance from the periphery of the egg. 
4) The authors do not figure any of their 4-celled eggs, of which 
they secured four along with other stages, although they present 
drawings of six 2-celled and other eggs. 5) The photographs 
