52 EDWARD MCCRADY, JR. 



VII. THE SEVENTH DAY 



Stages 14 to 17. The late bilaminar blastocyst. The 

 primitive streak. The mesodermal crescent. 



Stages 14 and 15 



The late bilaminar blastocyst. In the first half of the 

 seventh day the blastocyst enlarges from a diameter of 

 0.75 mm. to one of 1.0 mm. During this enlargement all of the 

 ectoderm gains in thickness, but the endoderm remains as thin 

 as it became during the fifth and sixth days. The medullary 

 plate lies in immediate contact with the shell membrane. Its 

 cells are quite crowded and are high-cuboidal in shape. Their 

 cytoplasm is granular, and very chromophilic. All of these 

 peculiarities make it a well-marked structure, conspicuous in 

 whole mounts or sections. The rest of the ectoderm is some- 

 what less sensitive to injury by fixing agents than it was when 

 it was thinner. It is in close contact with the small amount of 

 albumen left between it and the shell. This albumen disap- 

 pears as the vesicle enlarges at its expense, and consequently 

 the vesicle gradually regains its spherical shape. The nuclei 

 of the endoderm still appear larger than those of the ecto- 

 derm (possibly because they are flatter), and still stain very 

 deeply. Both layers are in close contact at all points. 



It should be mentioned that occasionally some endodermal 

 cells in a restricted region near the margin of the medullary 

 plate become noticeably larger than the others. The signi- 

 ficance of this change is not definitely known. Hartman ( '19, 

 p. 86) suggests that they mark the region above which the 

 primitive streak is soon to arise. It seems possible that they 

 represent the prochordal plate, a portion of which, the pre- 

 chordal plate, will later be seen proliferating mesoderm. 



During the second half of the seventh day the last traces 

 of albumen disappear. The vesicle is again perfectly 

 spherical. When the diameter is about 1.4 mm. the medullary 

 plate in a small circular area becomes thinner, so that by 

 transmitted light this region appears as a light spot near one 



