38 EDWARD MCCRADY, JR. 



eight to the A-group. Since each of these cell divisions is 

 slightly unequal, there is a fairly evenly graded series of cell 

 sizes from large to small. I do not mean, of course, that 

 they are lined up according to graded sizes ; actually, the two 

 B-cells which we saw in mitosis at the 12-celled stage form 

 four small cells at the B-pole near the remaining two un- 

 divided and therefore large B-cells. The two smallest cells 

 are at the opposite pole. The intermediate ones cannot be 

 dependably distinguished into A and B groups. Furthermore, 

 the last large cells at the B-pole soon divide, making it im- 

 possible immediately thereafter to be sure where the B-pole is. 



There are three factors which together make the further 

 tracing of cell lineage impossible: 1) the daughter cells from 

 each division are slightly unequal; 2) all the cells become so 

 small that the inherent margin of error in the wax plate 

 method, introduced by the square corners of the individual 

 wax sections, becomes proportionately too great for the 

 method to be practical; 3) the cells begin to resorb yolk and 

 to grow, so that daughter cells attain the size of their parent 

 cells before they divide again. Hartman also found it im- 

 possible to recognize any polarity or to identify the individual 

 blastomeres immediately after the 16-celled stage. 



However, before two more cleavage waves have passed 

 (i.e., in the 50- to 60-celled ova) a new polarity appears, which 

 is permanent. It is naturally of interest to inquire whether 

 the original polarity of the ovum is related to the secondary 

 polarity of the blastocyst, and, if so, how. The evidence bear- 

 ing upon this point will be discussed in chapter V. 



V. THE FOURTH DAY 



Stages 9 to 12. The unilaminar blastocyst. The endoderm 

 mother-cells. The new polarity. The primitive endoderm. 

 The medullary plate. The beginning of expansion of the 

 vesicle. 



Stage 9 



The unilaminar blastocyst. No true morula is formed. It 

 was pointed out in the last chapter that at the 4-celled stage 



