DEVELOPMENT OF THE OPOSSUM 21 



ovum may again dimly be seen in the center. The concentric 

 lamination of the albumen is apparent in the fresh state and in 

 sections. 



d. The pronuclear stage 



The eggs of lots Nos. 54 and 52, among which specimens of 

 the pronuclear stage are found, are apparently young eggs which 

 have just entered the uterus. The shell of batch No. 54 is 0.001 

 mm. in thickness, that of No. 52, 0.0034 mm. 



The pronuclei at first lie to one side of the egg and are sur- 

 rounded by an almost yolk-free area of considerable size (fig. 

 4). To this extent the egg at this stage possesses an obvious 

 polarity. Later, however, the yolk-free cytoplasm surrounding 

 the pronuclei occupies approximately the center of the egg (fig. 

 6), and here the first cleavage spindle will form (figs. 7 and 8). 



Egg No. 54 (2) d (fig. 4) was sectioned in such a manner that 

 both pronuclei lie in the same section. The specimen appears 

 normal in every way except for a third body situated between 

 the pronuclei and in contact with one of them, as if extruded from 

 it. This body has a structure similar to that of the pronuclei, 

 and contains a little chromatin substance. Figure 6 is made 

 from a section containing one pronucleus and the second pro- 

 nucleus was inserted from its position in the sixth section beyond 

 (sections 6 fx thick). In another case, No. 54 (2) a, the male 

 pronucleus seems to be in the act of withdrawing from the sur- 

 face as if the spermatozoon had just entered; for the pronucleus 

 is top-shaped with the point at the extreme surface of the egg. 



Neither the first nor the second polar body has been recognized 

 with certainty in these eggs. This is perhaps due to the inter- 

 ference of yolk granules, not thoroughly bleached of osmic acid 

 stain, and to the presence of numerous epithelial cells from the 

 tuba, imbedded between the laminae of the albumen. 



The eggs in the pronuclear stage do not exhibit any marked 

 polarity aside from the temporary position of the pronuclei. 

 Sometimes, it is true, the yolk is more or less concentrated in 

 large granules toward the pole opposite the pronuclei; in other 

 cases, as noted, the polar bodies are centrally located. An 



