YOLK AND YOLK-ENTODERM IN EARLY EMBRYO. 



97 



sections the pronephric duct appears, at first only on the left side, as an ectodermal 

 keel, beginning about the plane of the 8th somite. Thence, passing backward, it 

 merges with the somatopleure at about the plane of the 12th somite, after mm. In 

 this section the subnotochordal rod appears for the last time. In 00 the notochord 

 dips into the dorsal wall of the gut; and in pp it forms an evagination of its wall. 

 QQ and RR are sections through the neurenteric canal, and ss to uu through the 

 tail end. 



Two further details of this stage are shown in figures 74 and 75. The 

 former of a section close to that of fig. ']i ll, the latter from a section close to fig. 

 73 G, representing only a detail of the extra-embryonic blastoderm lying under the 

 region of the head. Fig. 74 has been given to illustrate the ingress of yolk material 

 through the ventral wall of the gut, for here is seen the wedge of yolk protruding 

 through the thickened mass of j^olk-entoderm cells, but under conditions which 

 bespeak the complicated nature of the process. For the rest, there is here not a 

 mere rupture which admits the yolk into the cavity of the gut, but an attendant 



jjiH y' yrx 



Fig. 74. — Detail ot section of early embryo shown in fig. 73 LL. 

 .V, Yolk plug pressing into cavity of gut ; y'. 3/", !/"', layers of yolk of different consistencies. 



series of changes of which the "rupture" itself is, with fair probability, the terminal 

 member. Thus the wedge-shaped mass of yolk ( j-) is composed of fine yolk; it 

 next passes through a transitional zone (r') into the coarse yolk (>'")• And on 

 either side of the wedge lies a layer of very coarse yolk (j'"), which obviously comes 

 into close physiological rapport with the neighboring layers, for this thickens as it 

 approaches the }'olk-wedge, and here it is filled with nuclei of extraordinary size. 

 Indeed on one side (left) we note that this layer of coarse yolk is separated from the 

 yolk-entoderm by a la^-er-like offshoot of the fine yolk (j-' ) from near the point of 

 the wedge. We observe also the relation which the bordering yolk-entoderm bears 

 to the point of the yolk-wedge, for this layer is here many times thicker than in 

 neighboring regions. The yolk-wedge, in short, which passes into the cavity of 

 the gut stands in specialized relation (i) to the usual mass of yolk, /. e., spreading out 

 fan-shaped below, thus securing a large surface of contact; (2) to the lateral areas 

 of coarse yolk; (3) to the lateral masses of yolk-entoblast, and (4) finally, as we 



