60 CARL GOTTFRIED HARTMAN 



9) The first cleavage plane divides the egg into approximately 

 equal halves. At this time the elimination of yolk takes place. 

 This occurs, not by the extrusion of a deutoplasmic mass at one 

 pole, but by the elimination from the entire periphery. The 

 two blastomeres form a new membrane just within the yolk- 

 laden submarginal layer, leaving masses of cytoplasm rich in 

 yolk between the blastomeres and the zona pellucida. In this 

 condition the eliminated material is later found in the cleavage 

 cavity of every stage until absorbed. The two blastomeres 

 exhibit no evidence of polarity. The yolk remaining within the 

 blastomeres in the 2-celled, as well as in later stages, occupies 

 a submarginal zone as in the unsegmented egg (figs. 7-10, pages 

 22-24). 



10) The second cleavage plane is again meridional and at right 

 angles to the first. The four resulting blastomeres are spherical 

 and shift their position until the pairs come to lie at right angles 

 to each other. From this time on the cleavage is indeterminate, 

 as in the Eutheria (text fig. 1 and figs. 11-18, pages 24-28). 



11) As cleavage proceeds the cells migrate to the periphery 

 and become closely appressed against the zona pellucida or, if 

 this has disintegrated, against the albumen layer. As a result, 

 at the end of the fourth cleavage (and sometimes before this) 

 the cells are arranged in the form of a sphere enclosing a con- 

 siderable cleavage cavity, in which the eliminated yolk and co- 

 agulum are to be found. The cells of the 8- and 16-celled stages 

 are practically alike in every particular (text figs. 2 and 3, figs. 

 19-24). 



12) A study of an unbroken series from the 2-celled to the 

 18-celled stages, together with a fairly complete series up to 

 the time of entoderm formation, makes it highly probable that 

 the cells of each hemisphere are lineal descendants of one or the 

 other of the two blastomeres of the 2-celled stage. The cells 

 of the two hemispheres are destined to form the embryonic 

 and the non-embryonic regions, respectively, of the blastocyst. 

 This interpretation is supported by a full series of wax models 

 of the cleavage stages. Hence, also, the unsegmented opossum 

 ovum possesses a potential though concealed polarity and con- 



