30 



CARL GOTTFRIED HARTMAN 



by the zona, for a large egg may contain small blastomeres. It 

 is due rather to differences in quantity of yolk and cytoplasm 

 eliminated in the previous stages; the more yolk thrown off the 

 smaller the blastomeres. The end result is, however, the same, 

 for the yolk is eventually absorbed and used as nutriment, 

 whether it remains within the blastomeres or not. The yolk 

 remaining within the blastomeres is arranged peripherally, as 

 in previous stages, and indeed in the 8- and 16-celled ova as well. 

 The peculiarities of the first two cleavage stages of the opossum 

 egg and their interpretation as presented above are fully con- 

 firmed by the succeeding cleavage stages now to the discussed. 



THE 8- TO 16-CELLED STAGES 



a. Third and fourth cleavages 



1) General statement. Eggs of five to eighteen cells were fur- 

 nished by specimens Nos. 85 and 117; these two batches in- 

 cluded five 8-celled and eight 16-celled eggs. Every other num- 

 ber of blastomeres from five to eighteen inclusive was represented 

 in different specimens. The eggs of No. 85 are to be considered 

 perfectly normal, for the female furnishing them was fresh from 

 the field and the eggs are of normal size (figs. 19-24). Yet the 

 eggs of batch No. 117 appear normal in every respect except 

 as to size (A and B text fig. 2). The reduction in size is to be 

 expected in view of the fact that a single ovary of female No. 

 117 discharged 43 or more eggs at the second oestrus period, 

 having one month previously discharged about twenty. The 

 measurements of several eggs of both batches (measured from' 

 sections) are given in the following table : 



TABLE I 



