VESICLE GROWTH AND IMPLANTATION 113 



in culture of whole vesicles during the period when the 

 albumen and zona coverings still remain is extremely diffi- 

 cult for the ova soon degenerate and often collapse (Water- 

 man, 1932, 1934). As soon as the early primitive streak stage 

 is reached, explantation results in a moderate degree of de- 

 velopment. Waddington and Waterman (1933) explanted the 





\ 



,!^ ' 



/ ■^-. 



Fig. 30. Camera lucida drawings of embryonic 

 areas of the rabbit at the stages of explantation. 

 XG, late pre-primitive streak. XE, stage of pos- 

 terior thickening. XL, medium primitive streak. 

 XK, pre-somite. XB, three somite, p.st., prim- 

 itive streak; c.pl, chorda plate; p.kt., primitive 

 knot; p. pi., prochordal plate; p.m.s., pre-meso- 

 dermal somite; s., somite. (From the Journal of 

 Experimental Biology.) 



embryonic portion of the blastodermic vesicles upon a me- 

 dium of chicken plasma plus chicken embryo extract and 

 found that the older and more differentiated the embryo at 

 the time of explantation the greater the degree of differenti- 

 ation in culture. Using the five stages illustrated in Figure 30, 

 the development observed was as follows : 



(a) The stage of late pre-primitive streak gives no appar- 



