EXTRA-EMBRYONIC BLASTODERM. 



79 



The complexity of the foregoing conditions (fig. 70) applies as 

 well to the yolk region as to the blastoderm itself. Without enter- 

 ing into undue detail we may note the following: Extreme vacuo- 

 lization of the subgerminal region (the vacuoles at the right in 

 the figure are indicated by dotted lines); they usually occur 



in or in association with the lighter areas 

 of the germinal yolk. If we regard the 

 vacuoles in the earlier stage as retaining 

 the character of intercellular spaces, they 

 have by this time undergone, in part at 

 least, change of function, serving now as 

 nutriment purveyors to the yolk ento- 

 blast. In this connection we find that at 

 various points, 10, the coarse yolk is 

 traversed by fine yolk in rifts, whose 

 shape suggests that of the vacuoles of 

 earlier stages. In this fine yolk, more- 

 over, many nuclei are present, and, judg- 

 ing from numerous amitoses, dividing 

 rapidly. In addition to these rifts of fine 

 yolk, we note that there occur at many 

 places throughout the coarse yolk small 

 areas of fine yolk, 1 1 ; these have in nearly 

 every case nuclei in or near them, and 

 we have thus ground for regarding the 

 yolk region of the egg not as a syncytium 

 pur, but rather as a mass of yolk-filled 

 cells whose boundaries have broken down, 

 but whose individuality as cells has not 

 yet been wholly lost. 



A second section of the extra-embry- 

 onic blastoderm of this stage is shown in 

 fig. 70 A, a detail of a section passing 

 through the blastoderm considerably in 

 front of the preceding section. Here is indicated even better 

 than before the presence of giant cells which have arisen from 

 the yolk, migrated outward, and are undergoing division in the 

 region immediately below the ectoderm. At one point (a) a 

 yolk cell of gigantic size is shown (unshaded); at other points 

 (b, b, b) similar yolk cells are undergoing division by amitosis. 



,, 



Figs. 70 A and B. -Sections of stage of early embryo figured in fig. 69. 



A. Portion of extra-embryonic region in section corresponding to fig. 69 B. 



B. Embryonic and extra-embryonic regions in section similar to fig. 69 C. 



