42 CARL G. HARTMAN 
f. The origin of the crossed arrangement of the first four 
blastomeres 
Since in the Eutherian ova there is very little yolk to be 
eliminated, even in cases, such as the bat, where the phenome- 
non has been described by Van der Stricht, the blastomeres of 
the 4-celled stage fill the space within the vitelline membrane 
rather snugly. It has therefore been suggested that mutual 
pressure is responsible for the shifting of the blastomeres and 
that in the crossed arrangement they occupy the minimum space 
in the egg. A glance at the specimen photographed in figures 6 
and 7, plate 3, will convince one, however, that this mechanical 
explanation is inadequte, for certainly here one cannot speak 
of mutual pressure of the blastomeres, for they are not even in 
contact, and yet in such eggs the shifting also takes place. 
Hence we must look for other causes of the shifting movement. 
It is, of course, quite possible that there is no shifting at all, 
but that the cleavage planes cut the two blastomeres of the 
2-celled egg at right angles, as has been suggested by Professor 
Hill (10, p. 31). According to this assumption, one of the 
blastomeres would be divided meridionally, the other equa- 
torially, and the crossed arrangement would obtain from the 
beginning. Indeed, a study of the 3-celled eggs described above 
would seem corroborative of this view, for here the definitive 
arrangement has already manifested itself. But two facts make 
this theory untenable. First, a number of 4-celled eggs and 
one 3-celled egg I find to deviate less than 180° from the parallel 
arrangement; hence for these one would under the theory have 
to postulate a backward shifting toward the parallel postion. 
But conclusive evidence on the point is furnished by eggs 
Nos. 306 (1) and 306 (2), in each of which both blastomeres are 
in mitosis. In the latter the spindles are exactly parallel. as 
shown by lines connecting their ends in D, text figure 4. In 
the former egg (A and B) the spindles in the blastomeres are 
36° removed from the parallel. These observations seem to 
indicate that division begins in both blastomeres of the 2-celled 
