40 EDWARD MCCRADY, JR. 



0.003 mm. and 0.004 mm. All of the eggs roll or float about 

 freely in the uterine fluid. Hartman's litter no. 314 (e.g., 

 W.I.C. 16220), removed 3 days after copulation, are type 

 specimens for stage 9. 



Stage 10 



The endoderm mother-cells. During the second half of the 

 fourth day when the total number of cells in the blastocyst 

 is between fifty and sixty some extraordinary events occur 

 which were first noticed by Selenka (1887) and then more 

 completely worked out by Hartman ('19). 6 Certain cells in 



Fig. 13 Photograph of section through 16284, showing endoderm mother cell. 

 From Hartman ('19). 



one-half of the blastocyst which may be called the endoderm 

 mother-cells (Urentodermzellen of Selenka) begin to enlarge 

 enormously (stage 10 and fig. 13). When I first examined 

 them I wondered whether they were just cells of an earlier 

 generation which had remained large while their neighbors 

 became reduced in size by continued division. But a study 

 of the volumes of the cells of the cleavage specimens showed 

 that these endoderm mother-cells are larger than any of the 



8 Hill ('10) denied the morphological significance which Selenka had attributed 

 to the cells about to be described here, but at that time Hill had not seen any 

 Didelphys preparations, and his opinions were based only upon his data from 

 Dasyurus. Hartman ('16) following Hill's lead, made the same denial, but later 

 ( '19) he confirmed Senlenka's interpretation and gave a very complete and perfect 

 account of the origin of the endoderm. 



