EMBRYOLOGY 



20 1 



egg. In a vertical section along a line which will afterwards 

 become the axis of the embryo, the Avhole headfold is in the shape 

 of an 8- The authors named above ingeniously suggest the 

 making of a rough model in order to render the somewhat compli- 

 cated matter easier to comprehend. Spread a cloth out flat to 

 represent the blastoderm, and by placing the left hand underneath 

 it mark the axis of the embryo, and then tuck in the cloth from 

 above under the tips of the fingers. The fingers, covered with the 

 cloth and slightly projecting from the level of the rest of the cloth, 

 will represent the head, in front of which will be the semicircular 

 or horseshoe-shaped groove of the headfold. 



A similar, but shallower fold, appears at the hind end of the 

 embryo. This, the "tailfold," travelling forAvards and the "headfold" 

 gradually extending backwards, and a pair of lateral folds uniting 

 the two and moving inwards, ultimately succeed in forming a 



A.K 



A?n. 



Transverse Sections through the Trunk ok an Embrvo on the Third and Sixth Days. 



A.F. Anterior amniotic fold ; Al. Allantois ; Am. Amniotic cavitj-; Ch, Chorda dorsalis ; 



m, Spinal marrow ; Se. Jlembrana serosa. 



tubular sac seated upon and connected by a continually-narroAving 

 hollow stalk, Avith that larger sac which is formed by the extension 

 of the rest of the blastoderm over the Avhole yellow yolk. The 

 smaller or upper sac contains, or rather forms the embryo, the 

 larger or lower sac is the yolk-sac. As incubation proceeds the 

 contents of the yolk-sac are gradually assimilated by nutritive 

 processes into the tissues forming the growing Avails of the embry- 

 onic sac. Consecjuently the latter becomes larger and larger at 

 the expense of the former. Within a feAv days of the hatching of 

 the chick, Avhen the embryo is nearly complete, the j^olk-sac is still 

 of some considerable size, and is slipped into the body of the 

 embryo through the umbilicus or navel. In the article Altrices 

 it has been iiointed out that in the Nidifugje a considerable 

 amount of this yolk still exists Avhen the embryo is hatched, Avhile 

 in the Nidicolse this food-yolk has been completely, or nearly so, 

 used up by the time the embryo is ripe. 



The Avhole mass of the white of the egg, betAveen the shell and 



