CHANGES BETWEEN 24 AND T^T^ HOURS 57 



aseptic precautions the eggs were opened and the first somite 

 marked, in some cases by injury with an ''electric needle" 

 in other cases by the insertion of a minute glass pin. Following 

 the operation the shell was closed by seaHng over the opening a 

 piece of egg shell of appropriate size. After being again in- 

 cubated for varying lengths of time the eggs were reopened. In 

 all cases the injured first somite was still the most anterior 

 complete somite. All the new somites except the incomplete 

 ''head somite" had appeared caudal to the first pair of somites 

 formed. 



The Lengthening of the Fore-gut. — Comparison of the rela- 

 tions of the crescentic margin of the anterior intestinal portal 

 in embryos between 24 and 30 hours shows it occupying pro- 

 gressively more caudal positions (Fig. 27). This change in the 

 position of the anterior intestinal poital is the result of two 

 distinct growth processes. The margins of either side of the 

 poital are constantly converging toward the mid-line where they 

 become fused with each other. Their fusion lengthens the fore- 

 gut by adding to its floor and thereby displaces the crescentic 

 margin of the portal caudad. At the same time the struc- 

 tures cephalic to the anterior intestinal poital are elongating 

 rapidly so that the portal becomes more and more remote from 

 the anterior end of the embryo with the further lengthening of 

 the fore-gut. 



As a result of these two processes the space between the sub- 

 cephalic pocket and the margin of the anterior intestinal portal 

 is also elongated (Fig. 27). This is of importance in connection 

 with the formation of the heart for it is into this enlarging 

 space that the pericardial portions of the coelom extend and 

 in it that the heart comes to lie. 



The Appearance of the Heart and Omphalomesenteric Veins. 

 Although the early steps in the formal ion of the heart take 

 place in embryos of this range, detailed consideration of them 

 has been deferred to be taken up in connection with later stages 

 when conditions in the circulatory system as a whole are more 

 advanced. 



In dorsal views of entire embryos the heait is largely con- 

 cealed by the overlying rhombencephalon (Fig. 18) but it may 

 readily be made out by viewing the embryo from the ventral 

 surface (Fig. 19). At this stage the heart is a neaily straight 



