44 EDWARD MCCRADY, JR. 



They are, then, larger, in one dimension and definitely smaller 

 in two others, and their actual volumes have not been studied. 

 Personally, I feel that the source from which all this theori- 

 zation springs namely, the idea that the first two blastomeres 

 represent in any significant way the 'embryonic' and 'non- 

 embryonic' portions of the later vesicles is unsound. In the 

 next few T pages I shall suggest a totally different interpreta- 

 tion of the polarization of the later blastocysts. 



' Fig. 14 Photographs of stage 11 (Hartman, '19). A, living ovum in 

 Ringer's solution. B, section through primitive endodermal cells and medullary 

 plate. 



Stages 11 anl 12 



The primitive endoderm. In the 1-celled stage the ovum 

 measured about 0.11 mm. in diameter. No enlargement at all 

 occurs until the endoderm mother-cells begin to form. The 

 principal changes up to this time have consisted of the ex- 

 trusion of most of the deutoplasm, the segmentation of the 

 protoplasm proper, the orientation of the segments or cells 

 along the zona leaving the yolk balls in the center, the lique- 

 faction and resorption of most of the yolk. These are changes 

 in the distribution and condition of materials contained in the 

 original unicellular ovum. 



When the endoderm begins to differentiate, changes which 

 involve the acquisition and utilization of new materials are 

 initiated. Such changes are reflected in the growth of the 



