FORMATION OF UERMINAL LAYERS IN CHELONIA. 27 



opening of the blastoporic passage. This section also shows 

 that the roof of the well-defined area which forms the lower 

 opening of the blastoporic passage is formed by the chorda- 

 entoblast and that the latter thus occupies by itself a special 

 recess of the digestive cavity. From just where the 

 chorda-entoblast and the darm-entoblast join each 

 other, there goes out laterally on each side a string 

 of cells (mes.) placed dorsally to the darm-entoblast 

 and ventrally to the ectoblast and distinct from 

 both. This is the commencing mesoblast. The meso- 

 blast is therefore not continuous from the first across the 

 median line. 



The surface view of the next stage is represented in fig. 3. 

 The embryonic shield has become pear-shaped, the broader 

 end being the front end. The blastopore is horseshoe shaped, 

 as in figs. 2 a and b, and occupying its concavity is the yolk- 

 plug. The head-fold has just begun, and is seen as the pos- 

 terior of the two semilunar curves found near the front end of 

 the embryonic shield. The anterior curve is probably the 

 commencing amniotic fold. Between the head-fold and the 

 blastopore there is seen in the median line an opaque streak, 

 which is narrowest in the middle, and becomes broader ante- 

 I'iorly and posteriorly. This is the chorda, which is nearly 

 completed in the middle, but still unfinished toward each end. 

 The area pellucida is, as before, found only toward the front 

 and the sides. The pear-shape of the embryonic shield seems 

 to have been produced mainly by its posterior part having 

 lengthened. 



Figs. 18 — 23 are selected from a series of cross-sections 

 obtained from an embryo of this stage, and are arranged from 

 behind forwards. 



Fig. 18, the most posterior section represented, goes through 

 the lateral limbs of the horseshoe-shaped blastopore and the 

 yolk-plug occupying its concavity. The ectoblast, which con- 

 sists of only a single layer of cells at the sides, becomes gra- 

 dually thicker towards the median line, which it does not, 

 however^ reach. At a short distance from the latter, and at 



